Error codes: You have exceeded the limit of license assignment for your account.

Each license is intended to be assigned to one user indefinitely. While it can be removed from one user and assigned to another, there is an undocumented limit to the number of times a license can be reassigned. Once that that has been reached, Zoom gives you this error: “You have exceeded the limit of license assignment for your account”. Once you purchase an additional license and assign it to a user, my understanding is that the one or more of the remaining locked licenses will be released to be reassigned. 

It may be a violation of Zoom’s Terms of Service to freely reassign a small number of licenses among a larger number of users in an account. 
https://explore.zoom.us/en/terms/