RE: User unable to login to Dynamics 365
There are two different processes involved:
1) Sign in to your AAD account
2) Login to D365 with that AAD account.
First one needs to be succesful before second one can be attempted.
The "cannot access your account" is not related to D365FO so it should not be used if the user simply can't login to D365FO.
This process should be only run if the user can't login to their account at all (not able to access emails etc).
Could you share the exact error messages that the user sees when "the system kicks out the user saying its not a valid user."
Also, as others pointed out, if the user is from a different domain than the "native" domain of the system, the user setup must be done a bit differently.
If the user is from your own tenant, perhaps the account was for some reason deleted and recreated in Azure Active Directory, changing the unique technical identifier of the user account. In this scenario you would need to delete the user from D365FO users list and add them again.