@afifhaziq I'm using Opera GX Desktop, but Opera Browser on my Android. Signed-in on both devices. I just know that going to opera.com/connect on my Desktop, then on my Android, navigating to the bottom-right Menu->Flow->Connect opened up a QR code scanner. I pointed that at my Desktop displaying the QR code (opera.com/connect) and it worked for me, albeit I was getting a different error code than you.
No idea why that's happening though, m8 ¯_(ツ)_/¯
I recommend you download the default Opera Browser on your phone, sign-in, then follow my steps above to connect to Flow. Just to see if it works. It could be a bug in the mobile Opera GX. I like the default Opera Browser over the Opera GX mobile, but you do you.
Best of luck, m8