Semantic Error (SEM1198): Assignment to step status is not allowed.

The semantic analysis has detected a write access to a member of the step state structure SFC_STEP_STATE. This structure applies at the output of an SFC FB and can be used to poll diagnostic data on the state machine execution. It consists of the following members:

ERROR_CODE (UDINT)
ID (UDINT)
INTERLOCK_FAULT (BOOL, indicates active interlock)
T (TIME, indicates elapsed execution time)
X (BOOL, indicates step activity)

As these are status information they may only be read not be written in the code.

Remedy

Double-click the error message to jump to the affected code position.
Delete the write access to the SFC_STEP_STATE structure member.

Further Info
For more information refer to the topic "SFC Parameters, Operating Modes and Data Types"