Press "use now" then scroll up and press "view QR code"
I just did it and it didn't ask me for a code.
There must be an issue with your account so you'll need to contact O2.
All numbers are in this guide Guide: Coronavirus Community Help and Support
Or 202 from your O2 mobile.
Advice from an O2 Guru to get through:
When you call 202, you will be asked for the reason you are calling.
Stay silent for 15 seconds and you will be taken to the keypad options.
Press 2 for Everything Else
Press 5 for More Options
Press 7 for Anything Else which will put you though to someone.
You can also try saying "Upgrade" or "Fraud" to be connected to an advisor.
Track usage, upgrade and get support with the My O2 appDownload My O2