I’ve had the same problem on macOS 26 with Opera. Here’s what helped me:
Sign out and back in to your Opera account.
Check sync settings under Settings → Browser → Synchronization. Make sure the data types you want to sync are selected.
If you’re using a sync passphrase, try resetting it via Opera’s account page.
On Mac, some users fixed it by removing “Opera Safe Storage” from Keychain Access and restarting Opera.
If none of these steps work, the easiest way forward is to reach out to Opera support. They can give advice tailored to your setup and help get your sync working properly again.