A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Either you should have Grouped/selected multiple sheets and doing it. Just ungroup the multiple sheets which you can do by selecting any sheet/right click/ungroup sheets.
Or
To make sure there are no merged cells in the entire source sheet, click the top left corner of the sheet or press Ctrl+A twice. Then choose Home, open the Merge & Center drop down and choose Unmerge Cells. If there were any hidden rows or columns containing merged cells, this would address those.
Also go thru this thread