If your YouTube account is not part of a Google Account, you have a legacy YouTube account. Only older YouTube accounts (created before May 2009 and haven't been used since 2011) may be legacy YouTube accounts.
Since Legacy YouTube accounts are not part of a Google Account, you need to sign in with the YouTube username instead of your email address. Just enter the username into the Email field on the sign-in page. Once you sign in, you'll immediately be asked to add your YouTube account to a new or existing Google Account:
A YouTube account today must be part of a Google Account to be used. If you do not want to add your YouTube account to a Google Account, you can also close it by clicking on Account Settings below the form.
If you sign into YouTube with your Google Account but don't have a channel on that account, you can also add your legacy YouTube channel to your Google Account by signing into the account and visiting the claim legacy channel page.