Semantic Error (SEM1278): ENO variables must be 'VAR_OUTPUT'.

In an included C# library, a POU contains an invalid declaration of an ENO variable. Like each other POU output formal parameter, the ENO output must be declared with the VAR_OUTPUT attribute.

Remedy

Double-click the error message to jump to the affected variable.

Contact the provider of the library.

If the library source code is available and can be edited:

Correct the variable declaration by specifying the correct attribute.
Release the library again.