I was creating a Windows Virtual Desktop environment for a customer today and had some difficulty when I was creating the host pool. WVD is not yet fully integrated with the Azure portal so doesn't support dynamic groups. You have to add a comma separated list of users when you are creating the host pool (or you can do it afterwards). Anyway I was getting this error when I entered a pre-prepared list of approximately 200 users.
The list may only contain UPNs (in the form of valid email addresses) separated by commas. Leading whitespace, trailing whitespace, or additional spaces before or after commas is not allowed.
I looked through my list in Notepad trying to find a space before or after a comma but no joy. Eventually I found the problem. One of the users had an apostrophe in their UPN and this was messing with my formatting. Removing this UPN from the list allowed me to continue.
It's the little things that cost you the most time sometimes. I hope this helps if you see a similar problem.
Until next time.....