Hi,
I have faced a similar situation.
There are two possible causes. One is Permission and the other is License. I think the license is the most likely cause.
If you can press the Create New Company button but get an error, it is probably a permission issue. If the button to create a new company exists, but you can't click it, I think it's a license issue.
The cause may be different from yours, but in my case, the cause was an expired license. Since I was a D365 administrator and my license had expired, I could sign in but could not create a new company. Other users who are not administrators could not sign in.
I hope this is helpful to you.
S.Kawamura