Illegal operation: abs( {concave} )

why the following sentence is wrong? Illegal operation: abs( {concave} )

Help for cvx/abs

Disciplined convex/geometric programming information for abs:
abs(X) is convex and nonmonotonic in X. Therefore, when used in
DCPs, X must be affine. abs(X) is not useful in DGPs, since all
log-convex and log-concave expressions are already positive.

Your argument of abs is not affine.