OneDrive is able to download documents from SharePoint site but will not upload, errors out

The successful downloads show that OneDrive can read the library, but they do not establish that its connection can upload. Site ownership alone does not rule out a library configuration that makes synchronized content read-only. First capture the exact OneDrive error and check whether File Explorer shows a lock icon. Confirm OneDrive is using the same work account that can create files in SharePoint. Test uploading one small file through the browser and through the synced folder. Then open the affected library’s Settings > Library settings. Use Permissions for this document library > Check Permissions to verify the user has Contribute or higher. Review Versioning settings for required checkout and restricted draft visibility, then check required columns and validation settings. Microsoft documents these as possible sync restrictions. Preserve business controls; have the library owner approve any change. If those checks are clear, provide the error text and comparison results.