Go to Apps, and look for accounting. Click More and select upgrade. wait for a while and it should be fixed.

if it does not work,
go to Accounting > Configuration > Management > Accounting Reports. Look for Profit and Loss, locate the Less Costs of Revenue and drag it into the gross profit, you can make it as the child of the gross profit, or you can also put it back to the original position (basically you just move it, and for some reason the error is gone)