This is covered in the guide that @Enlli has referenced, but just to highlight that your first top-up on O2 PAYG needs to be done using a voucher purchased from a shop. 👍
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)