Error message to change to a paid plan without exceeding 20 clients

Morning, we are not exceeding 20 clients however receive the message that we should upgrade to a paid account. I tried to delete old clients without success and we are not even close to having 20 clients. There are still plenty of old invoices archived. Do you expect me to delete all of those too before I can use the free version again? Please advise!

I found the solution by myself. It is required to purge deleted clients again after deleting them to free up space.