If you're having trouble signing in with your YouTube username, try signing in to your account by entering your full email address. Then, enter the same password you’ve been using to sign in to YouTube.
You're still signing into the same account as always—you just need to enter the email address on the account instead of your YouTube username.
Why do I need to sign in to a Google Account on YouTube?
You access YouTube with your Google Account. The password you've used to sign in is your Google Account password.
As we integrate services across Google, we no longer support signing in to Google Accounts with YouTube usernames. Instead, you need to sign into YouTube with your Google Account email address (this is often Gmail, but can also be a different domain like Yahoo, Hotmail, etc.). Learn more about using your Google Account for YouTube.
But I don’t like my Google Account email address!