Can't Log Into Sandbox Accounts

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.

After a couple of days, my logins magically started working. There must be a delay between creation of sandbox accounts, and synching up with Apple Pay systems. Not sure what other explanation there could be; I was certain I was using the correct passwords, and they simply didn't work until they did.

Can't Log Into Sandbox Accounts
 
 
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