Hi En-Tie,
As you encountered an issue when you uploading a document to SharePoint online, my suggestions are as below for your reference:
1. Create a new simple document (.docx) file on your machine.
2. Create a new library and ensure you have the permission to upload files.
3. Try to upload the file again.
4. If the issue persists, please try to use Open with Explorer with IE10 to upload files and see if the workaround performs fine.
Thanks,
Maggie Li