You're trying to add a new Event Manager to your event, in the "Other organizers" field, but your colleague doesn't show up in the list.
This is usually because that colleague isn't part of the same team as you.
How to fix it?
Only an organization owner can add permissions and change your colleagues' teams. The solution is therefore to contact one of them: no other role can do this instead.
To find the organization's owners, click on your profile in the top right of the administration center, then "Edit my account":
Our support team remains available by email at support@eventdrive.com.