Just thought I’d let you know that you aren’t alone (I knew I’d seen this problem before), PayPal is a bit picky when it comes to adding the prepaid card. Here is a link to the previous topic on this, maybe it will help if COPs can’t see a problem at their end.
https://community.monzo.com/t/unable-to-add-monzo-card-to-paypal-wallet/6703
Just to summarise some of the issues there, some were due to an incorrect address, others were due to PayPal not accepting the prepaid card as the sole payment method in your PayPal account (because it’s a prepaid card). Might be worth checking it out and seeing if any of those causes are the same in your case.
Hope you get it sorted.