Hi,
Thank you for writing to Microsoft Community Forums.
We understand your concern as you are unable to sign-in to Microsoft RD web client.
In this scenario, we would suggest you to sign-in to this portal and check: https://rdweb.wvd.microsoft.com/webclient
If the issue persists, then let me help to point you in the right direction. I would suggest you to post your query in TechNet Forums where you will find professionals with expertise on Microsoft RD web client and that would be the appropriate forum
For additional information, please refer this article: Set up the Remote Desktop web client for your users
Regards,
Aditya Roy
Microsoft Community – Moderator