can't complain
Things are fine. A casual response to questions like "How are you?" or "How've you been?" A: "Hey, Pat, how are you?" B: "Ah, can't complain!"
nothing to complain about
Things are fine. A casual response to questions like "How are you?" or "How've you been?" A: "Hey Pat, how are you?" B: "Ah, nothing to complain about."
(I) can't complain. and (I have) nothing to complain about.
Inf. a response to a greeting inquiry asking how one is or how things are going for one. Sue: How are things going? Mary: I can't complain. Mary: Hi, Fred! How are you doing? Fred: Nothing to complain about.