Error in OP4A: "Only enter valid values for PAPLAN"

Hi All,

When creating shift definitions using transaction OP4A, below error occurs. Please let me know how to resolve this.

'Only enter valid values for "PAPLAN"

Message no. SV530

Diagnosis: You have entered a value which is formally correct, but not allowed in this context.

System Response: The input value is not accepted.'

Thanks in advance.