Dear Ahsan,
Thank you very much for your more details.
The field status of Posting key is in client level, it will effect to all company codes. => Posting key is not the reason.
I have tested with same cases and find that, the reason is in Changing rule.
I have 3 documents:
- 1 Invoice: 1800000000 => Reason code can be changed
- 1 credit memo with invoice rereference 1600000000 => Reason code cannot be changed
- 1 credit memo without invoice reference1600000001 => Reason code can be changed
Screenshot:
Same with down payment, so, please check your business requirement for change document rule whether you need to tick on 2 options or not.
- No Invoice-related Credit memo
- No Credit memo from Down payment.
If not, untick them...you can change the reason code.
Julie