A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Hi!
Welcome to Microsoft Q&A forum.
This is a very common issue in Microsoft Excel and the good news is it’s usually something simple.
Kindly do a test and open a new sheet and manually type 10, 20, and 30 and do an auto sum below it. If that works on a new sheet that means previous data could have formatting issues.
Check as well if cell is converted to text instead of numbers.
Let me know once done so I can assist you further.