Can't sign in with YouTube username
Instead of signing in to your account by entering your YouTube username, enter 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 into a Google Account on YouTube?
You may not know it, but every time you've signed into YouTube since 2011, you've been signing into a Google Account—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 can no longer support signing into 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.).
But I don’t like my Google Account email address! If the email address isn't Gmail, you can change your email address. Or, you can replace it by adding Gmail to your account. If you already have Gmail, you can add an alternate email address to sign in with.