Dear Nisha,
You can double click at the error and you will be able to see the long text of the explanation as below:
NA GP591
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Short Text
The master record for allocation cost element & was not found
Diagnosis
The allocation cost element has been entered in the activity type .
However, a valid master record for this allocation cost element was not found for the last day of the planning period.
System Response
Processing was cancelled.
Procedure
Correct the inconsistency by
recreating the allocation cost element , or
entering a different allocation cost type in the master record of allocation element
So it should be the cost element is not valid for the current planning period. You can check this first in KL03 for the activity type to see the "Allocation cost elem". Then you can use KA03 to see the validity period of the cost element.
Best regards,
Jason