Cannot Change User type from Basic to Licensed

I have one Zoom Pro Subscription account and am managing one user, including myself.

When I try to change one user from Basic to Licensed, I encounter an issue. Previously, I was able to make this change, but today, after changing all users to Basic, I can no longer upgrade a user to Licensed.

I received the following message: "You have exceeded the limit of license assignment for your account. Buy more licenses to increase the limit."

However, my account shows that one Zoom Pro license is available, yet I still cannot make the change.

Please help me resolve this issue.