A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Dear Blake 21,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
As per your description, please let us cooperate and please don’t worry, we can work together to narrow down and resolve the situation.
Per you error message "your file wasn't uploaded because your changes can't be merged with changes made by someone else.” this may happen occasionally if someone uses an Excel feature that is not fully supported in Excel co-authoring yet. This can stop the co-authoring process temporarily. So, the way of users to open this workbook may not cause this error. It probably relates to the Office desktop version of users’ end. For your reference: Collaborate on Excel workbooks at the same time with co-authoring
At present, we suggest you click "Save as copy" to store all the changes and go to SharePoint Online and modify the original files manually. However, we would like to troubleshoot this issue for you.
1.Does the issue happens on all files or a specific one?
2.What is your Office version: To check version: Open Excel>File>Account>Product Information (Please mask your private message for protecting your private message)
3.Are you using OneDrive sync client app on your machine.
Your understanding and patience will be highly appreciated. I hope that you are keeping safe and well!
Sincerely,
Waqas Muhammad