How to find missing userId when trying to add request participant

Hi, I am trying to add a request participant but I am getting this error: Sorry, but the following usernames could not be added as participant to this request: ug:6b39c59c-40ca-4e75-95b5-a416774b8930

In the request body I have supplied an array containing our main account’s accountId. However, I am not sure what this accountId refers to, I also don’t know why its looking for that specific user that it cannot find for this call. Thanks in advance!