A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Possible answers:
You are currently editing a cell
The worksheet is protected
Possible solutions:
Format the cells (shortuct Ctrl+1), go to alignment, check the merge cells box.
Then select center horizontal alignment.
Or, avoid merging cells (it can cause so many problems...) and use the "Center across selection" option for horizontal alignment.