How can I add two or more file attachments into the %Net.MailMessage

First, should be so (AttachFile):

s status=msg.AttachFile("F:\MyDir","myFirstFile.pdf",1,,.count)

s status=msg.AttachFile("F:\MyDir","mySecondFile.pdf",1,,.count)

Second, you did not specify the error text.

Try the following simple MAC-example, replacing the values with your own:

#include %systemInclude
new

try{

  $$$AddAllRoleTemporaryInTry
  new $namespace
  
  set msg=##class(%Net.MailMessage).%New()

  set msg.Subject="Subject"

  set msg.From="from@domain"

  do msg.To.Insert("to@domain")

  do msg.TextData.Write("Hello!")
  
  $$$ThrowOnError(msg.AttachFile("F:\MyDir","myFirstFile.pdf"))
  $$$ThrowOnError(msg.AttachFile("F:\MyDir","mySecondFile.pdf"))

  set smtp=##class(%Net.SMTP).%New()
  set smtp.smtpserver="123.145.167.189"

  $$$ThrowOnError(smtp.Send(msg))
      
}catch(ex){
  write "Error ", ex.DisplayString(),!
}