Hi Tamsin,
I am able to import the footer on my test site:

That means there’s no issue with the template file. The issue is on the site you are importing the template. Let’s do basic troubleshooting first. You could try testing for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time. This is important because a pluging might have javascript error that is preventing the importing process.
The site where you are copying the template is showing NOT FOUND error. I cannot access it. If it is not cause by a plugin, please share correct URL and admin credentials inside a secure note. Thank you.