In a report, in the detail line the text box is "=[field1]+[field2]" and in the grouping line, i had used "=sum([field1]+[field2])" (in both the cases quotes have not been in the file but only to clearly identify them here". While it works in the grouping line, in the detail line it shows the error message "Error".
Also, if you have already added two fields together within your query, you don't need to add them again in the report. Just name a field and add the fields within that field, and then use that field as the control source on your report.
Not sure what else is going on in your DB but if it works in the GroupHeader/Footer it should work in the DetailSection. Check your spelling if you Typed in the FieldNames. That is a very common thing to happen and so easily overlooked. Also check for Spaces too. I think you have it correct if in the DetailSection you have:
And in the Footer you say it works correctly so that's fine. So check out your Spelling and possible Spaces. The best way to avoid that is to Create it in the ExpressionBuilder by DoubleClicking on the Fields instead of typing. It happens, trust me.
If that and what LaurelLee has suggested doesn't work I would suggest Zipping and Posting your database for someone on here to fix your problem for you.