How to fix this error OneDrive Account Already Connected' Issue on MacBook

You can fix the "Account already connected" issue. 

First remove OneApp and all its installed system files and folders with AppCleaner.
Then delete the OneDrive folder under /Library/CloudStorage/ with this Terminal command: sudo rm -R "/Users/YOURUSERNAME/Library/CloudStorage/OneDrive-Personal". 

After this I was able to use the standalone OneDrive app.