A320 FCTM AO-020 P 6/18:
The use of the autopilot is STRONGLY recommended. Following an engine failure, the rudder should be trimmed out prior to autopilot engagement.
Once AP is engaged, the rudder trim is managed through the AP and, hence, manual rudder trim command, including reset, is inhibited.
I.e. put the AP in, we can all fly but it frees up capacity.