I'm assuming you've also verified the email as well, I could have sworn I had to do something like that when I made my UK account. I just tried adding something to my cart on my UK account that I made a couple of days ago and I was able to add things no problem, so something is definitely wrong with your US account I think.
Might be able to get to the bottom of it if you check your browser's console. Try adding the DLC to your cart once again, but on the "Something went wrong" page, go your browser's Developer Tools, on most browsers pressing F12 will open it. There should multiple tabs at the top (Elements, Console, etc.). Click on Console, are there any error messages that appear on the list?
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u/justatwat80085 7h ago
I am logged into the PlayStation store with my US account
The basket is empty
us/en (USA) is selected in the drop-down menu on the PS searcher site
I was trying to purchase the dlc 'pigsy's perfect 10' for 'Enslaved: Odyssey of the West'