Oh, this is a misunderstanding. I thought, the screenshot is coming from a (not showed) link, and anticipated the link points to some ISC site.
Anyway, ChatGPT and other ChatBots (nowadays, they often pop up on various sites, mainly on those of big companies) tries to mimic a human and often end up only with an reference to FAQs or with a wrong (inappropriate) answer.
They all base upon AI (some says: AI=artificial intelligence, others say: AI=absent intelligence). My bottom line is, AI is for some areas already "usable" and for others, it still "will require some more time".

You can't access the class name alone

write $system.Version          // is just a class name, something like
write ##class(SYSTEM.Version)  // (hence both lines gives you a syntax error)

You have to specify a property or a method. Because $System.Version is an abstract class, you can specify a method only

write $system.Version.GetVersion()

Hope this clarifies things

To be honest, I was also impressed, but not about ChatGPT rather about the suggested solution!  I have no idea, who at ISC wrote this recommendation, but the five clicks on my IRIS-2021.1 end up in NOTHING. On the right-side panel I see 'System Information' but this can't be clicked, below is 'View System Dashboard', here you see everithing, except a version info. So dear ChatGPT, if someone asks you again, the (better) answer is:
- login to management portal
- click on 'About' (here you see the version and much much more)
This works for all IRIS versions (until now) and for all Cache versions which have a Management Portal. For the older (over 20 years) Cache versions right-click on the cube and select 'Config Manager' (on Windows, of course. In that times, I didn't worked on other platforms, hence no advices).

First, the suggestion of Victor Gordillo  is correct.

do ##class(Some.Class).%BuildIndices()

solves most index related problems.

Second, there are a couple of things, we do not know, but you
- did you made an in-place-upgrade or a clean install + data transfer
- the class in question is the same on IRIS and Cache (i.e. no change was made)
- is the class re-compiled (on IRIS)
- what do you exactly mean when you say "Not opening an instance...": (1) you get an NULL-OREF or (2) you get an error
- what do you provide as a NULL value: (1) "", (2) $c(0), (3) " " or something else as open parameter

A shortened class definition (we don't need the whole definition) would be also helpfull, something like this

Class Problem.Class extends %Persistent
Property Field1 as %String;
Property Filed2 As %String;

Index composite on (Field1, Field2) [ Unique];

If your serial class is named Data.Serial (as in your example code) then you should use the same name for the serial property too

Class Data.Persistent Extends %Persistent
Property MPID as %Integer;
Property Name as Data.Serial;   <--- !!!!!

The correct way to set the values

Set Obj=##Class(Data.Persistent).%New()
Set Obj.Name.FirstName=FirstName   ; <----
Set Obj.Name.LastName=LastName     ; <----
Set tSC=Obj.%Save()

I think, I have a solution for you

ClassMethod GetImage()
	s req=##class(%Net.HttpRequest).%New()
	s req.Server="www.distrelec.de"
	s req.SSLConfiguration="SSL"
	s req.ReadRawMode=1        //  <<---- this is your solution
	d req.Get("/Web/WebShopImages/landscape_medium/_t/if/sortimentsboxen-1.jpg")
	q req.HttpResponse

To get the image

s rsp=##class(Some.Class).GetImage()
i rsp.StatusCode=200 {
    s file="c:\temp\imageName.jpg"
    o file:"nwu":0
    i $t u file d rsp.Data.Rewind(),rsp.Data.OutputToDevice()
    c file

 That's all...

To download that image, you need just a few lines of code

Class Some.Class Extends %RegisteredObject
ClassMethod GetImage()
	s req=##class(%Net.HttpRequest).%New()
	s req.Server="www.distrelec.de"
	s req.SSLConfiguration="SSL" // use your SSL-Config-Name
	d req.Get("/Web/WebShopImages/landscape_medium/_t/if/sortimentsboxen-1.jpg",1)
	q req.HttpResponse

So your code is more or less OK, but the rest of the process is ominous

set rsp=##class(Some.Class).GetImage()
zw rsp
rsp=7@%Net.HttpResponse  ; <OREF>
+----------------- general information ---------------
|      oref value: 7
|      class name: %Net.HttpResponse
| reference count: 3
+----------------- attribute values ------------------
|    ContentBoundary = ""
|        ContentInfo = "charset=UTF-8"
|      ContentLength = 17759
|        ContentType = "image/jpeg;charset=UTF-8"
|               Data = "8@%Stream.GlobalCharacter"
|Headers("CACHE-CONTROL") = "max-age=0"
|Headers("CONTENT-LENGTH") = 17759
|Headers("CONTENT-TYPE") = "image/jpeg;charset=UTF-8"
|    Headers("DATE") = "Mon, 14 Nov 2022 10:57:10 GMT"
|    Headers("ETAG") = "a919e895229c7883864aecbfa2717516"
|Headers("LAST-MODIFIED") = "Thu, 01 Jan 1970 00:00:01 GMT"
|Headers("SET-COOKIE") = "visid_incap_2373370=1U8JalxbRJOzRFviQpi05AYfcmMAAAAAQUIPAAAAAADRm"
|Headers("STRICT-TRANSPORT-SECURITY") = "max-age=31536000; includeSubDomains; preload"
|   Headers("X-CDN") = "Imperva"
| Headers("X-IINFO") = "5-19652015-0 0CNN RT(1668423430526 51) q(0 -1 -1 0) r(0 -1)"
|        HttpVersion = "HTTP/1.1"
|       ReasonPhrase = "OK"
|         StatusCode = 200
|         StatusLine = "HTTP/1.1 200 OK"

The sender says, content type is "image/jpeg", which is OK, but charset=UTF-8 is, I think, a problem. A jpeg-image usually starts with (hex) bytes:

FF D8 FF E0 00 10 4A 46 49 46 ...

The HTTP-Response gives us

do rsp.Data.Rewind()
zzdump rsp.Data.Read(10)

3F 3F 3F 10 4A 46 49 46 00 01                           ???.JFIF..

But I'm in no way a web-expert, but it seems to me, Cache tries to decode (according to content-type =image/jpg; charset=UTF-8) the incomming raw (jpeg) data. The first byte, FF, will already give an error (no utf-8 encoded byte can start with FF) and returns an "?" char as a replacement. The next two "?" (hex: 3F) chars are also arised from (inpossible) decoding. Why the same page works, if you try it with Chrome or Firefox: I think, they either ignore the charset=UTF-8 or just show the raw data after the first decoding error.

OK, no punctuation, no empty words, etc.... My lowest bid: 74 chars

ClassMethod Order(s)
	f  s p=$p(s," ",$i(i)),w=$tr(p,1/17),$p(z," ",$tr(p,w))=w ret:p=w $g(z)

By the way, the following three variants all have the same size of 74

	f  s p=$p(s," ",$i(i)),w=$tr(p,1/17),$p(z," ",$tr(p,w))=w ret:p=w $g(z)
	f{s p=$p(s," ",$i(i)),w=$tr(p,1/17),$p(z," ",$tr(p,w))=w ret:p=w $g(z)}
1	s p=$p(s," ",$i(i)),w=$tr(p,1/17),$p(z," ",$tr(p,w))=w q:p=w $g(z) g 1