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 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 PLCnext Info Center which provides many information and practical user tips on the PLCnext Technology control platform and PLCnext Engineer.

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