You must add a new platform before you can enter your domain name in the "App "Domains" field, if not, it will not save and it will display the error message below:
This must be derived from Canvas URL, Secure Canvas URL, Site URL, Mobile Site URL, Page Tab URL or Secure Page Tab URL.
To fix the problem, click on the "+Add Platform" tab on the App Settings page. Select "Website" if you want to get App ID for your site or blog. Thereafter, you will see the "Site URL" and "Mobile Site URL" fields where you can enter your site URL as seen below:
After saving the settings, you can then enter your domain name in the "App Domains" field as seen above. If you now save the settings, it will save successfully.