Note this is referring to doing this via a computer, or the site, note through the youtube app. However, the steps should be similar, even in the app.
Try the following...
1. Log into your youtube account.
2. Go to the settings (gear icon).
3. No go to "View additional features" and then "Verify".
4. In here click the "Verify" button.
5. Specify your 'country location', choose a 'verification method', and then click "Next".
6. Once you get it, enter your verification code and then click "Submit".
7. Then finally click "Continue".
You can also get more info here... https/support.google.com/youtube/answer/6060318?hl=en