A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
The fact that Excel is in Compatibility Mode is not what prevents most changes from being made. It simply means that the file is saved in an old file type which doesn't support some of the newer features of the program. Those features cannot be used.
Use Save As to save a copy in the current .xlsx file type [or .xlsm if it contains macros] then close & reopen the file to see if conditions improve. If not, it may be that some form of Protection is applied in the file.