Hi,
Each posting rule may be defined in both posting areas: 1 for G/L posting and 2 for subledger posting.
If you don't define the posting rule in posting area 1, you will get message 'not to be posted' in FEBA - G/L posting.
If you don't define the posting rule in posting area 2, you will get message 'not to be posted' in FEBA - subledger posting.
If you need only one posting, the 'not to be posted' should not be considered as an error.
I suppose you don't use the same posting rule in all lines, so you have the 'not to be posted' message in some lines and not in the others.