Adalo only supports use of a Custom Domains that include a Subdomain. i.e. app.mydomain.com or www.mydomain.com.
If you are trying to set up your domain to go to a Root Address such as mydomain.com you'll need see if it's possible to set up forwarding or a redirect from your Domain Provider.
If your DNS provider offers CNAME flattening, ALIAS or ANAME records, you may be able to choose to redirect all traffic to your site to your root domain (e.g., yourdomain.com).
The following DNS providers allow you to point root domains to a CNAME/ANAME /ALIAS record: