Why can't I login to youtube through windows movie maker

Hello,

Welcome to Microsoft Community Forum. We would like to know more about your issue not being able to sign into YouTube within Movie Maker by answering the questions below:

• Can you sign into YouTube directly?
• Can you sign in to Youtube on Movie Maker using another Google account? (To check this, I would suggest that you create a test account)
• Can you try changing your Google account password?
• Please verify if 2 step verification is enabled for your Google account. You can check the settings by signing in here

We'll await for your response.

Thank you.

I believe YouTube is changing their account login to require you to use your email address rather than user name.  The following details are from YouTube's support pages.

Can't sign in with YouTube username

We no longer support signing into your account with your YouTube username—but you can still access your account:

Enter your email address instead of your YouTube username. Your account has an associated email address. If you're not sure what it is, see below.
Your password still works. Your password hasn't changed—it's still the same password you've been using to sign into 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.