We have AX 2012 R3 on-premis and are trying the setup Document management with a SharePoint online location.
I have configured as per the Microsoft guidlines but when i try to upload a file i get an error "Error during upload".
The user account im logged into sharepoint with has edit access on the library and can access it via the web browser no problem.
Your issue is one of two things most likely, you need a hotfix or you need to enable TLS 1.2.
Probably, your issue is most likely TLS 1.2 - https://learn.microsoft.com/en-us/windows-server/security/tls/tls-registry-settings but you may also need some hotfixes.
KB4599942 - This is probably the main hotfix you need. KB4035350 - This is older, but may be needed.