Semantic Error: '...' is not a valid function block.
In a graphical worksheet, a function block is used which is no longer available because e.g., the library where it is defined has been removed, or the user POU has been deleted in a 'Function & Function Blocks' folder in the COMPONENTS area.
In the textual code, a variable is used in the same way as a function block. In the error message, the data type of the misused variable is indicated. In the following error example, MyVar is declared as an INT variable. In ST, MyVar is used like a function block.
MyVar(a, Q=>temp);