Hi @Gert2198 welcome to the GLFW forum,
You can’t use member functions as callbacks unless they are static, as the signature is different for member functions (they require the object pointer to dereference).
You can use window user pointers to set the member pointer so you can then call your member function from a free function.
For example see What is a possible use of glfwGetWindowUserPointer? - #2 by tombsar