Is this a first top up?
If so, you need to buy a voucher for first top up.
If not, try clearing your browser cache or try a different browser.
Some ways to pay help here
http://community.o2.co.uk/t5/Discussions-Feedback/Ways-to-pay-guidelines/td-p/874982
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)