iPhone - "Authentication Failed" messaged when trying to log into iTunes and/or App store

Every time I try to log into iTunes or the App store with my iPhone, it gives me an error message: Authentication Failed. This is problematic because I can't install apps, and I can't install updates for my apps.

I am using the correct Apple ID. (There is zero chance it's wrong.. it's definitely correct.)

I am using the correct password. (There is zero chance it's wrong.. it's definitely correct.)

I tried restarting my phone.

I tried resetting my phone's network settings.

I tried signing in using my cellular data instead of Wifi.

I turned off the automatic date and time settings, verified the date/time was correct, tried to log into iTunes and the app store, got the same error message. Turned automatic date/time back on - it's still correct - and I still can't log into iTunes or the App store (same error message).

I have spent hours watching YouTube videos, reading help articles, and none of the advice given fixes the problem.

No matter what I try, I get the same error message: Authentication Failed.

Admittedly, as I'm sure you guys can relate, I'm getting peeved off. Apple charges premium prices because their products are supposed to be user-friendly and work. But this issue is the opposite of user-friendly. :(

If anyone knows how to fix this, I'd be super grateful.