Hi Kate,
I understand the level of frustration you are going through; I know it is hard to work when things don’t go as expected. I will try my best to help you with this issue.
Sometimes you may see this error when coauthoring with others. The error bar appears, AutoSave turns off, and no more updates come in. This error happened as your document cannot be saved to the server because your edits can’t be merged with changes from others.
If you have any edits you don’t want to lose, select Save a Copy and save the file under a new name. Reopen the live file and add back any changes that didn't make it to the server copy.
Note:You can also select all and copy, paste the document into a new window, and then selectDiscard Changes*. You can then add back any changes that didn't make it to the server copy.*
Besides, instead of opening the file from Teams via Open in App, try opening the workbook from within Excel app itself (File>Open>OneDrive/SharePoint location), see if you still encounter this error.
further, please check if your Office apps are fully updated. The latest version of Microsoft 365 for mac is 16.59 (22031300). To check version, open any Office app such as Excel, go to the Excel menu and choose About Excel.
If the version appears lower, then manually update Office apps, click Help > Check for Updates from any of the Office app to get the latest build.
Let me know if you still encounter this issue.
Regards,
Neha