Semantic Error: Assignments to read-only variables are not valid.

A write access to a read-only variable was detected.

Two possible reasons:

This variable is located in the read-only system memory (i.e., a controller-specific system variable),

or the variable is declared as input formal parameter of a user-defined function POU using the 'Input' declaration keyword (VAR_INPUT acc. to the IEC 61131-3 standard).

Note
Write access to input formal parameters of functions can be permitted by setting the compiler option 'Allow write on variables with usage input'. Select 'Extras > Options' and open the category 'Compiler | IEC Compiler Settings' and select the respective checkbox.

Read-only variables cannot be written.

Further Info
Please refer to the user manual of the controller for further information on system variables.