A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Try renaming the two sheets - use Sheet1 and Sheet2. It is possible there is a space in one of the names.
What I like to do is have two totally empty "bookend" sheets called Start and End; one before Sheet1 the other after Sheet2.
Then I use =Sum(Start:End!B2) and I can readily add more sheets between Start and End as needed.
best wishes