A top-level domain is one like mysite.com, mysite.net. and if you have the default mysite.ucoz.com you have a sub-domain.
They do not aprove it because of .ucoz.com, because your site somewhat counts as a part of ucoz.com instead of as your own site.
I am not completely certain how adsense works, but I guess having a subdomain interferes with the plan.
All you can do is get more content on your site, and buy and attach a domain to your site.
Post edited by Acid - Thursday, 2012-11-22, 5:43 AM