I'm in EU. I even followed this guide but Im still restricted to buy it.
That guide doesn't explain a basic thing that maybe you aren't following even if it's a common sense thing: the PSN user that is allowed to download for free in PS4/PS5 these type of games must be the same PSN user that bought it on PS3/PSP/Vita.
Maybe you have different users on each console, or maybe in the newer one you have multiple user accounts and aren't using the same one who bought it on PS3/PSP/Vita.
It really really sucks, because if I understand this all correctly, the PS5 is now finally backwards compatible with PS1, PS2, PS3 and PS4. The fans wanted this for years. Now it's here, and it's tied to an expensive membership. Fuck right off, Sony.
You're wrong, PS1, PS2, PSP, PS4 games can be bought, downloaded and played on PS5 without having any sub.
All emulated PS2 and PS4 games (and compilations / crossgen / ports / remaster / remakes of PS1+PS2+PS3+PSP+Vita titles) that were on PS4 if you bought them on PS4 you can download and play them for free on PS5, no game sub required.
All digital PS1 and PSP games released emulated on PS4 and PS5 are also available to be bought in the PS4 and PS5 PS Stores without needing to be subscribed to anything. And if you bought them digitally on PSP/Vita/PS3 you can download them for free on PS4 and PS5 without needing to pay them again or to have any sub.
As happened until now, PS4 and PS5 hardware can't emulate PS3 properly, so all the PS3 games it has to be bought, rented or downloaded to be played locally are the ones who also were released for PS4 or PS5 via crossgen releases, ports, remasters or remakes. So the ones that aren't PS3 native versions. There are plenty of these ones included in PS Plus Premium/Deluxe.
Then there are native PS3 games, that can be played on PS4 and PS5 only via cloud gaming subscription (can be bought/downloaded because can't be emulated/played locally), so these are the only ones tied to a subscription.
Then there's the exception of PS2 classics for PS3, which were different than the PS2 classics for PS4+PS5 that are the PS2 ones included in PS Premium or in the PS4/PS5 stores. If you bought a PS2 Classic for PS3 and they release that same PS2 Classic for PS4/PS5 you'll have to buy it again, they aren't crossbuy: they count as different games, because in addition to different emulation and feature new extra stuff like trophies.
It doesn't recognize my credit card.
If it doesn't recognize the credit card I assume the issue is that you're using the credit card of a country in a PSN user account from a different country, or that already is being used in other PSN user account. Or well, you could try by paying using other methods other than directly credit card.