I think it is linear (affine) but cvx thinks it is convex

[Miriam_C]

This is part of my formulated problem, and it gets this error:

I think C2 is an affine function, as the only variable is the matrix F, but why do I get this error?

[Michal_Adamaszek]

|x| is not an affine function. It is convex though, so you can upper-bound it (t\geq |x|) but not lower-bound it.