Microsoft Excel cannot access the file "/Users/imac/Documents/.localised'. There are several possible reasons:

AGM1949,

Thanks for the additional information. In addition to testing the issue in a new user account, I suggest opening Excel while in safe mode. Here's more information about this step: Use safe mode to isolate issues with your Mac

From there, you can also try uninstalling the app, then reinstalling it to further isolate the issue: Install and uninstall apps from the Internet or disc on Mac

Lastly, be sure your macOS version is compatible with the version of Excel you're using. If needed, update you Mac: How to update the software on your Mac

Enjoy the rest of your day. 

Hi AGM1949,

Thank you for posting in the Apple Support Communities. I understand that you’re intermittently receiving an error when opening a certain document in a third-party app on your iMac. I’d love to share some information with you that should help.

First, I recommend following each of these steps: If an app won't run or install on your Mac after you upgrade macOS. Ensuring your Mac is running the latest version and checking to see if the app developer has a new version for their app often resolves many unique issues. 

Also, consider whether this happens with only one file in particular, or if it happens when launching all files within Excel. Do you get the same error when creating a new document as opposed to launching the existing one? If not, that particular file might have an issue.

Additionally, I suggest trying to launch the file in a new user account. If the issue persists, try launching it in safe mode: Use safe mode to isolate issues with your Mac

I hope this helps get you started. Kind regards.