I'm guessing this is a permissions error. But everything I've tried enabling hasn't fixed the problem. Does anyone know why this is greyed out or how to fix it?
This is in ArcCatalog.
For me, this was caused by a schema lock (as suggested by Kirtep). I had edited the RuleID field separately in Python with an update cursor. Once I closed the Python window, I could edit the Representations again. So I suspect the feature class was used somewhere else (e.g., another application, another user), and had an active lock on it.