It's unclear what situation you're in. It appears that you don't have Image Processing Toolbox. I thought that im2gray() was part of IPT, but it's possible that I'm wrong. If you don't have IPT, you won't be able to use imcomplement. For class-agnostic image inversion see this answer:
Note that IPT and MIMT themselves are not equivalent, but in this case, IPT imcomplement() and MIMT iminv() are equivalent. Also, I should remind that the linked copy of iminv() requires neither MIMT nor IPT.