Is this happening with just one file or all your Excel files? Have you tried creating a new file, making changes to it, then saving? If that works, you might want to simply copy/paste the contents from the existing file to a new file.
And even so, I agree it's high time to upgrade to a later version of MS Office. If you use the MS Office applications frequently, I'd highly recommend an Office 365 subscription (yearly), which ensures you always have the latest versions, and you get either 1 user + 1 TB OneDrive storage (Office 365 Personal) or 6 users + 1TB OneDrive storage per user (Office 365 Home). Office 365 also includes free phone/chat support for as long as you remain a subscriber. However, if you don't want to go the subscription route, there's also Office Home & Student 2019 (one-time purchase - can only be installed on one Mac - does not include OneDrive storage and support is limited to 60 days).
https://products.office.com/en-US/compare-all-microsoft-office-products?tab=1