For ease of use on the internet, most people understand that you can simply type in google.com or www.google.com and both will go to the same place.
Now having both go to the same place isn't just standard internet behavior. You as a domain owner will need to set this up.
If you're not in the loop of how domains and registrars work in the background, we can safely say that for you, there's likely no reason you don't want your root (naked) domain and www subdomain to go to the same place.
Navigate to the domain you want to setup under My Domains and click the three dot menu and Manage DNS. Under Records, you'll be either adding an A record for www if it doesn't exist or editing your existing one.
If you don't see a pencil icon next to any existing records here it means you have forwarding turned on. You'll need to turn that off to proceed.
Regardless, you should have one record with the following:
Type - A
Name - www
Value - 159.203.146.41
TTL - 1 Hour
And a second with:
Type - A
Name - @
Value - 159.203.146.41
TTL - 1 Hour