ME51N error- with Account assignment as Q

While creating the Purcahse requisition with account assignement category as Q (Q-project make to order), balanace sheet account is updating.

and the warning message as "Do Not assign any objects in cost accounting to account 123456"  &message no KI281.

Plese be noted that in OBYC, for BSX  with valuation class and balanace sheet account maintained.

alreday huge data was updated in EBAN-KNTTP as Q.

Note: while creating in ME51N, after entering the data intially P&L account is displayed, but when i ENTER the hey, system is updated woith balance sheet account in grey mode.

How should we correct it?

should i create this balance sheet account  with CElement tpe as 90 for statistical any??

Regards

Pabbati Obulesu