My Pokémon Go account has locked me out and I can’t enter. This has happened very recently. Unfortunately, if I sign up as a returning player, I need my username and password, which I forgot. I would be able to get back my username if I entered the player ID and screen name, but I have no clue on what these two words mean! I really need someone’s help!
I looked it up on Google and according to Pokemon's support website screen name is only displayed in any public areas of the website (and in Pokemon GO it is used in game publicly so that other players can know who you are), unlike the username that is meant to log into your account. And as for the player ID, you have to generate your own specific player ID by logging into Pokemon's website. You can find more info about screen name here and about your player ID and how to generate a player ID here. You can also find info about how to change screen name here.
Hope this helps :)