Functions with return value not appearing in Expressions Dictionary

Basicaly what title says.

I'm trying to use a function to process some data and get the value directly in the return, but at the moment of call it simply don't exist in the expressions dictionary. If I force the machine by put "Functions.MyFunction" it doesn't allow me and shows the message: The function 'MyFunction' has a return type of 'None' so it can¡t be used as an expression.

Any idea what can be causing this?

I'm sure it has a return value

Edit: If I try to put the return in the main event of the function it also don't work.

Yup, the name of the function is that thing and not MyFunction, that was only for clear explanation of the problem.