One possibility is that you don't have the appropriate Service enabled to allow for this connection.  This is pretty easy to check, from the SMP go to:
System Administration > Security > Auditing

If auditing isn't enabled then Enable it and run your test again, then go into View Audit Database and see if there is an access error logged.

It is quite possible that you don't have the Call-in service enabled (I don't think it is by default in Normal or Locked Down instances).  From the first .Net documentation link you provided:

[quote]

If InterSystems IRIS has been installed with security option 2, open the Management Portal and go to System Administration > Security > Services, select %Service_CallIn, and make sure the Service Enabled box is checked. If you installed InterSystems IRIS with security option 1 (minimal) it should already be checked.

[/quote]

Check that out and report back if it solved your issue.

@Sean Brady - per "The push to use vscode over studio is there but the processes have not been migrated so we are still having to fall back to studio for deployments." ... FYI, when you use Embedded Git (or any Serverside Source Control toolset) you can use either VS Code and Studio (or both!).  The IDE triggers the behavior on the server so either should kick off the same behavior.

I am sorry but I can't answer your question about deployment manager as I have always relied on source control and automation tools based on that (I have been spoiled by that ;) ). 

If you are hitting merge conflicts I am going to go out on a limb and guess that these are in the Production Class?  If so then I highly recommend that you look at using Production Decomposition which solves this issue by allowing Embedded Git to manage PTDs rather than the Production Class as a whole - https://github.com/intersystems/git-source-control/discussions/519

We also have a standing weekly meeting for Embedded Git users to provide updates on features and 'office hours' for questions.  DM @Pravin Barton if you want to get an invite.  

@Sean Brady - welcome to the D.C.!  Great question and you should get a lot of help here.

First, definitely watch the video linked by @Oliver Wilms in the earlier response.  It will help you understand how the InterSystems-based healthcare platforms tend to work best with the embedded source control paradigm due to how changes are made in the Management Portal and not just in VS Code.  

As you try to get Git set up, can you please confirm that you using the Embedded Git package?  https://community.intersystems.com/tags/embedded-git

You said you are having issues working with Git - could you please provide more details?

If you haven't, you should review the Branching Strategy doc on GitHub for Embedded Git (https://github.com/intersystems/git-source-control/blob/main/docs/hcc.md#general).  While it is in the HealthConnect Cloud section, @Pravin Barton has assured me it is generally applicable.

While you are working towards getting a proper Git-based progression in place, you can use Deployment Manager in the Portal (but you should stop using this once you have Embedded Git working).  This is the easiest way to grab things in bulk and move them between environments.  

Please note that HealthShare Registries are not yet covered by Source Control / Change Control hooks, so you will need to move those by hand or via custom scripting.  Those will come later this year - see the Global Summit presentation on the topic here: https://www.intersystems.com/change-control-for-healthshare-intersystems/

Hopefully this is enough to get you rolling.  Let us know what questions you may have!