My goal is simply to test a Safari (macOS/iOS) Apple Pay integration in a sandbox environment. I created a sandbox tester account. However, when I try and login to this account anywhere (icloud.com, on a Mac in macOS settings, or on a phone or even the iOS simulator in iOS settings), I get "Your AppleID or password was incorrect." I'm 100% positive I'm using the correct sandbox AppleID and password, though. What am I missing??
The payment handling backend, merchant validation, and everything else has been set up. I'm simply trying to test payments with a test card, from a device logged into a sandbox iCloud account with a test card in Wallet.