Can't create another user license

Can't create another user license

Hi,

I'd like to add another user/license, so that I have a new, separate mailbox to my main one. I will pay for it, but I don't know how/where.

When I try to add a new user, I get this message:
Max user license limit reached. Please purchase/ugrade license to add more user(s) with mailboxes.

Do you know how to solve this? It's very important that I get this solved asap.

Thanks,