We tried to recreate the concern and here are the steps that worked in our end:
Open Excel
Click File > New
In the Search box type **Academic Calendar (any year)**and press enter
One Template is returned. Double-Click the example and click Create
Note: There is no footer in the spreadsheet created for me using this template.
Click Insert > Header & Footer > Chose any option from the drop-downs.
When you get back to the page you cannot “click” on the text in the header or footer.
The problem is the template has a Custom Footer that overlaps the header and footer.
Change it to narrow and you will see the header and footer are now clickable
If you have any questions, feel free to post it here and we'll be happy to assist you.