Semantic Error: Only variables can be assigned to VAR_IN_OUT parameters.
In a code worksheet, another element than a variable has been connected to a VAR_IN_OUT parameter ('Usage = InOut') of a function block. This is not allowed. Connect a non-constant variable to correct the error.
A formal parameter declared with 'Usage = InOut' is not a "real variable" but a reference to another variable. It can be compared to a pointer that points to the address of another variable.
This referenced variable is read and written by the FB. Writing, for example, a literal is not possible. Thus, only variables are allowed at in/output parameters.