Running Mono... C++ Compile Error

Hi,
I recently started to work on an iMac and have a problem with Compiling Code. I wanted to set my Characters MaxWalkSpeed like this: GetCharacterMovement()->MaxWalkSpeed = 10.0f;
But when I compile i get this screen, which I don’t understand the meaning of.

Because i used to work on Windows i compared to an older script I wrote. They were exactly the same but for some kind of reason it doesn’t work on Mac.
Thank you for helping.