It sounds like the worksheet might be protected, which would prevent you from editing the footer. Here's how you can check and remove the protection:

1. Click on the "Review" tab in the ribbon at the top of the Excel window.
2. Look for the "Protect Workbook" button in the "Changes" section of the ribbon. If it's highlighted, click on it to unprotect the workbook.
3. If the "Protect Workbook" button is not highlighted, click on the "Protect Sheet" button in the "Changes" section of the ribbon.
4. In the "Protect Sheet" dialog box, make sure that the "Footer" checkbox is unchecked. If it's checked, uncheck it and click "OK".
5. Now try clicking on the footer again. You should be able to access it and customize it as needed.

If the above steps don't work, it's possible that there is a glitch in the worksheet. In that case, you can try copying the contents of the worksheet into a new workbook and see if that resolves the issue.