There are quite a few problems, but the one being shown is that your code has
Wektor &operator=(Wektor &wek);
but the operations return a Wektor by value.
The compiler cannot transform a Wektor in a Wektor& because the result is a temporary and the parameter is a non-const reference.
Changing the parameter of assignment to
Wektor &operator=(const Wektor &wek);
will solve this issue (but like said before this code has quite a few other problems).
If you don’t understand why a temporary cannot be converted (bound) to a non-const reference then probably you should read first a bit more about C++ rather that writing code and trying to compile it. Just pick a good book and read it cover-to-cover first, things will be a thousand times easier.
C++ is not a language to learn by experimentation for quite a few reasons, no matter how smart you are (actually being too smart can worsen the situation because sometimes the correct answer is just plain illogical, for historical reasons).