@Mariusz Iwaniuk thank you very much for your reply. Unfortunately I'm not able to use your document. Whenever I run your document, I get the following error.

plot(simplify(tau(x)), x = 0*Unit(mm) .. 100*Unit(mm));
Error, (in Units:-Standard:-+) the units `mm` and `1` have incompatible dimensions

I have also tried the rewrite your method, but it was ended in the same error.

@ebrahimina thank you, but it wasn't quite the thing i was looking for. I would like to be able to this use a function, like tau, to give me kN at a certain mm without having to remove the units like you did. More specifically; my questions is whether I can keep my units on a function, and still plot it.