Once you enter your domain name and save, you will get a notification depending on your domain type, and your domain's status will appear as "Pending" in the Overview section. Above this section, you should see, in yellow, the reason why it is pending.
If you get the check your DNS configuration message, then click Check DNS (blue box).
You will get a pop-up with instructions like the example below:
After setup is completed and the Status is not pending anymore, you can activate the domain as your Main domain (red box).
This process could take between 24-48 hours. If you're still having difficulties and the pending status hasn't changed in over two days, feel free to contact our Support Team, who will be happy to help you.