There were three issues:
1) You were using a phi function without first declaring it with DefScalarFunction.
2) You have a sum of 26 objects on which to do various things. It helps to construct a list of those objects, manipulate them individually, and then add them.
3) When results start being not small, using Simplify may take longer than we'd like. It tries hard to get you a simple result. Using Together may give already good enough initial results.
See the attached notebook with comments at the end.