Hi Amy Chiu, thank you for reaching out. I understand you're using Microsoft 365 Personal and facing issues uploading files to M365 Chat via the desktop app, even though it works in the web version. I know how important it is to have a smooth experience on your laptop or desktop.
May i know what troubleshoot have you tried?
To resolve the upload issue in M365 Chat, ensure OneDrive is syncing properly and you're signed in with the same account; clear the app cache by deleting files in %appdata%\Microsoft\Teams or %appdata%\Microsoft\M365Chat, update the app via the profile menu, confirm both OneDrive and M365 Chat use the same Microsoft account, and if needed, unlink and re-link OneDrive through its settings to re-sync your files.
Best regards,
Lucky Charm