Following up on @hkeric.wci’s post …
Use a "The custom Code <expression> condition is valid" condition with the expression:
return (ttABCCode.Any(r => !r.Unchanged() && r.ABCCode.StartsWith("T")) ? false : true);
Or don’t use the ( ? : ) to reverse the result, and just change the second part of the condition line to invalid.
Tweaked as you need it.