Semantic Error (SEM1133): 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 machine 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
Also refer to the iCube Info Center which provides many information and practical user tips on the iC9200 Series control platform and iCube Engineer.

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