Diagnosis
No number range for settlement documents defined in controlling area RELC.
The settlement document is a document type in its own right, which is used for cancelling settlements and repeating month-end closes.
Procedure
Define a number range for settlement documents. Choose Maintain number ranges for settlement documents.
SOLUTION:
STEP 1: PATH--> CONTROLLING -->INTERNAL ORDERS--->ACTUAL POSTINGS--->SETTLEMENT---->

STEP 2: Press CTRL + F, to find the CO Area

STEP 3 : Select Element and click on Assign Element to Group

STEP 4 : Assign Number Range 01 > SAVE.

Thank You,
Rita Jha