Diagnosis
You want to post via a special G/L indicator which was not maintained completely in the system. The specification on which reconciliation account is to be updated in the general ledger is missing.
System Response
The line item cannot be entered.
Procedure
In system configuration enter a special G/L account corresponding to reconciliation account 200000 for account type K and special G/L indicator A.
SOLUTION:
STEP 1: Go to T Code FS00 and create a G/L account for Down Payment Made
STEP 2: Go to T Code OBYR > Double click on Special G/L indicator "A" and Provide your Chart of Account
STEP 3: Enter Reconciliation A/c GL (left side) and then Special G/L Ind (Right side) > SAVE
You can also refer below blog for more detailed explanation:
https://www.gauravconsulting.com/post/down-payment-in-sap-s-4-hana
Thanks,
Adharsh K S