You have a license for 3 users and have assigned the server as a host. This means that you can only have 2 workstation users + 1 remote user opening the same file at the same time. You cannot purchase additional seats for your existing license. You must cancel your license and purchase a new license of QB Desktop US Enterprise. Anyway, why did you need QB Desktop Enterprise? You can use QB Desktop US Premier to manage up to 5 users. You can still purchase a one time license of QB Desktop 2021 US Premier or 2020 US Premier at a lower cost if required.