Your outer pow_abs has an argument which is not affine. Hence the error.

help pow_abs

POW_POS Power of absolute value.
POW_POS(X,P) = ABS(X).^P.
P must be real and greater than or equal to one.

Disciplined convex programming information:
pow_abs(X,P) is convex and nonmonotonic, so X must be affine.
P must be constant, and its elements must be greater than or
equal to one. X may be complex.