Excel sheet is not allowing me to insert or delete columns, rows or cells.

First, check whether Insert/Delete are greyed out or clicking them produces an error.

If they’re greyed out, press Esc to exit editing mode, then right-click a sheet tab and choose Ungroup Sheets if available. Since you’ve already confirmed the sheet isn’t protected, try a new blank workbook next. If it works there, the issue is specific to the original file; if not, restart Excel and test again.

If you get an error message instead, please share its full wording—the fix depends on what it says.