First top up usually needs doing with a voucher from a shop as the online system won't recognise the number until after the first top up.
Scroll down this help thread for ways to top up.
https://community.o2.co.uk/t5/Pay-Monthly-Pay-and-Go/How-can-I-pay-my-bill/ba-p/1014090
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion.
This is not Customer Service so we are unable to help with account specific issues.Currently using:
Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)