Invalid Domain: This Domain is invalid?

I am very new to all of this. I’ve registered my domain and then signed up for the full Hosting. When I try to do the Once Click Install of Wordpress, it won’t let me put the .com on the end of my domain. Then when I hit the enter, it says Invalid Domain. Have no idea what’s going on here, but I’m trying to understand, lol. Help!

At no point during the One Click install do you type your domain name.

If you select the Simple Install you supply a subdomain name. So if your domain is mydomain.com, you select it from the drop down list and type a name in the box to the left, example, wordpress. You will then have a web address of wordpress.mydomain.com to access WordPress

With the Custom install you select your domain name from the drop down then type a folder name, for example, Wordpress. You will then access Wordpress at www.mydomain.com/wordpress

Hope this helps

Thank You for your help! I finally have gotten it figured out. Could not have done it without your help!