This is my problem
and the error message is
If real(G*BTSG_subset(m,n)*gain) <= 0, it looks like constant would be a sum of concave quadratic forms, hence allowable in the inequality constraints.
Something like (please fix up as needed):
X = f(:,n)'*at(:,n);
Then use X'*X, surrounded by the rest of the stuff, and make sure nothing complex multiplies X'*X without first applying real(...) to the complex stuff.
And in the future, please copy and paste code using Preformatted text icon, rather posting an image of the code.