Top-up vouchers are retired early this year.
https://community.48months.ie/t5/The-Knowledgebase/Voucher-Retirement/ta-p/52781What error do you get when you try to top-up online?
I don't remember any code being needed for online topups.
Who is sending this - 48 or your bank?
If it's 48 then contact 48 support for assistance (see the link in my signature below).
If it's your bank then contact them.
Hope this helps.