I stumbled around with this simple problem for longer than I care to admit.  Googling for a solution only found similar questions without any answers.  The problem occurs after adding a new user to the user database for trac (in my case, a simple .htpassword file), the user is unable to accept or to be assigned any tickets. Attempts to do so just generate the error message:
"<username> is not a valid value for the owner field."
The solution is to log in as that user, then click on ‘Settings’ in the top right, and fill out the name and email address.  Evidently after adding that, it saves the user information to the trac database, and makes it possible to use that user in other places.
We ran into this at work recently, and it seemed to be caused by enabling the “restrict_owner” flag. See the “[ticket]” section of https://trac.edgewall.org/wiki/TracIni and https://trac.edgewall.org/wiki/TracTickets#Assign-toasDrop-DownList