All in5 accounts come with root serial numbers and user serial numbers.
It's possible to activate in5 within InDesign using a root serial number (or an account creation code).
Doing so automatically generates a new user and it can therefore only be done when there is an open user slot (i.e., your account comes with 2 users, but you've only created 1 so far).
Once the user slots are in use (even if they're not all activated), the "user not found" or "account does not exist" message may appear when trying to activate in5 within InDesign.
If you need to, you can remotely deactivate or delete the user in your online account: https://ajarproductions.com/account
You can also activate with the user serial (or email/passcode combo).
Click the "..." next to the user to access the user serial number.
If you need more users, you can contact us to upgrade.