Oppo requires an official "Deep Testing" or "In-Depth Test" APK to initiate the unlock.
The Oppo servers haven’t fully "whitelisted" your IMEI/Serial Number despite the app saying "Application Approved." the serial is not match fastboot unlock verify fail oppo
If you have tried all the above and the error persists, it is likely that Oppo has patched the exploit for your specific firmware version. In this case, you may need to wait for a newer version of the Deep Testing tool or a community-made exploit. Oppo requires an official "Deep Testing" or "In-Depth