If you manage database(s) for a customer, it's not enough to manage it/them in SAP Business One. You must also delete the registration(s) of the respective database(s) in the portal.
Please log into the portal: https://portal.boyum-it.com/Account/Login and find the customer.
Then, find the correct license and click on the number of databases in the license:
A new window opens:
Depending on the license type and extra purchased databases, the total number of databases is showed as "Allowed". "Active" refers to the number of active databases.
Click the "x" to delete. Once done, press "Save" /"Close". There is now extra space for new databases.
NB: Please note that partners can't manage databases for their internal partner licenses. In case you need anything edited, please contact firstname.lastname@example.org for assistance.