In the case of blogspot users, when buying and using a custom domain in Blogger/Blogspot it won’t allow adding a domain without the top-level . Example, www.yourdomain.com is the acceptable one but yourdomain.com will be usually rejected.
So, the solution is URL redirection. The steps are fairly easy that won’t take a minute to configure and work.
1. First login in your blogger account
2. Go to Settings menu then, click Publishing tab
3. Now, you will see the Advance Settings when you’re publishing in a custom domain it looks like these:
4. Below the “Your Domain” field put check on the Redirect box
5. Don’t forget to type the CAPTCHA and hit SAVE before leaving.
6. To test, type your domain without WWW in an open browser. Congrats! You’re done.
Hope it helps for newbies...:)