Hi Paul,
To narrow down root causes of the issue, could you try the following steps to troubleshoot?
1. Have you installed Office Excel application on your computer? If so, I suggest you perform an Online Repair first to check if it helps.
2. Which browser are you using? If you use IE browser, could you check IE settings to see if the SharePoint Open Document Class add-on is listed under the All add-ons section with the status Enabled as shown below?
3. Did the error occur in other browsers, such as Chrome and Firefox?
4. If the issue persists, would you mind uninstalling Office and re-installing it?
In addition, here are two relevant threads for your reference:
Can't edit documents in SharePoint online
User is unable to edit excel file
Feel free to post back with any updates. Thanks for your efforts.
Regards,
Alice Wang