While creating service entry sheet and adopting the service the following error will be noticed as shown below for some cases.I have been intended to write this article where I arrived the same situation
Go to T-code :ME22
* Before that check whether release has been affected ,in such case un release the PO.
Input the PO number and select the service line item and click the account assignment and the Popup screen will appear and click the yellow arrow button.
Then check the following,if any indicator has been set in the Deletion flag or Account Assgnt Block . In such case remove the indicator and save it.
This will solve the issue and you can create the service entry sheet without any issue.