You will likely receive an error message along with two required CNAME records:
Blog CNAME: Name: www; Destination: ghs.google.com.
Security CNAME: Name: XXX; Destination: XXX. ("XXX" means this value is different for each person and is specific to your blog and your Google Account.)
After copying these two CNAME records, continue with the steps below to configure your domain on Dynadot.
Select "My Domains" from the left-side menu bar and click "Manage Domains" in the drop-down.
Check the box next to your domain name(s) and click on the "Action" button.
Choose "DNS Settings" from the "Action" list.
On the DNS page, select "Dynadot DNS" from the top drop-down menu.
In the "Domain Record (optional)" field, add 4 record lines, select "A" from the drop-down menus and enter the following four Google's IPs in the "IP Address / Destination" box.
216.239.32.21
216.239.34.21
216.239.36.21
216.239.38.21
In the "Subdomain Records (optional) field", add 2 record lines, select "CNAME" from the drop-down menus and enter the below two CNAME records you have obtained within your blogger account.
Subdomain: www → IP Address / Destination: ghs.google.com
Subdomain: XXX → IP Address / Destination: XXX (Please use the exact CNAME values provided in your Blogger settings.)
Click Save Settings to apply the changes.
Finally, return to your Blogger account and verify that the two CNAME records have been correctly added. Once verified, try saving the custom domain settings again to complete the setup.
Please be aware that it might take some time for your name server changes to propagate.
You can also create your blog right here on Dynadot! Upgrade to our Pro website builder plan to create a beautiful website with blog functionality.
NOTE: Your settings should look similar to what is shown below: (click to enlarge)
The requested URL / was not found on this server. That’s all we know.
i search about that and found this
A-records (optional) The following action links your naked domain (example.com) to your actual site (www.example.com). If you skip this step, visitors who leave off the "www" will see an error page. Enter your domain name in the format example.com, and list the I.P. addresses shown below in the "A" section. You'll need to create four separate A-records which point to four different Google IPs.
After you create your custom domain at Blogger, please follow these steps:
How do I create a CNAME?
A CNAME record is short-hand for Canonical Name record, which allows you to assign a domain or subdomain to another address or hostname. CNAME records should only be used if you can't use an A record.
NOTE: If you already have an A record within the Domain records section, please SKIP step 5!
Select "My Domains" from the left-side menu bar and click "Manage Domains" in the drop-down.
Check the box next to the domain name(s) and select "DNS Settings" from the "Action" drop-down menu.
On the new page, select the "Dynadot DNS" option from the drop-down menu.
In the "Domain Record (optional)" section at the top, select "Forward" as the record type, choose forward type, and enter "https://www.yourdomainname.ext" as the "Target Host". (NOTE: This is optional and please replace the value based on your own domain name.)
Under the "Subdomain Record (optional)" field, enter "www" as the subdomain, select "CNAME" as the record type and enter the CNAME record given to you.
Scroll down to the bottom of the page and click on the "Save Settings" button to save your changes.