You are trying to sync a OneDrive for Business (SharePoint) library that requires modern TLS 1.3 or HTTP/2 features. Solution: High Sierra supports TLS 1.2, but some enterprise endpoints require TLS 1.3. There is no fix for this. Consumer OneDrive (Personal/Basic) works more reliably than Business on 10.13.6.

: Microsoft no longer tests functionality on macOS 10.13, so bugs or sync errors will not be investigated or fixed. Microsoft Community Hub alternative cloud storage apps

Use this guide as a temporary bandage. Use the savings from not buying a new Mac to either purchase a third-party sync tool (Mountain Duck) or invest in a cheap NAS (Synology DS120j) to act as your sync bridge. Or, consider finally retiring that 2011 MacBook Pro.

: If your hardware supports it, upgrading to macOS 11.0 (Big Sur) or newer is the only way to regain full official support and security updates.

This is a challenging request because . Microsoft officially dropped support for High Sierra in March 2022 .