Hi SantaFeG,
To clarify the issue, I'd like to collect the following information:
1. Do you mean that you've created the users and assigned licenses to them, but the users didn't show in the "mailboxes" page of the Exchange admin center?
2. Have you tried sign in with those users' credentials to see if you can sign in successfully?
2. Is the status of the Exchange services in the service health page normal?
Sign in with the admin credential -> In the "Admin" page, click "service health"
Thanks,
Aaron Yuan