Russian nouns have several forms. «Ка́мень» is the nominative case form, it’s used for the subject of the sentence (among other things). «Ка́мня» is the genitive case form, it’s used after «нет» to express absence (among other things).

You can see all the forms in a declension table on Wiktionary: here (click on the «Declension of ка́мень» box to show it). The table might look intimidating at first, but don’t worry: you’ll learn all those forms in bite-sized chunks as you progress through the course.

This is another useful site: http://starling.rinet.ru/cgi-bin/morphque.cgi?flags=endnnnnp

Enter any form of a word and click view and it will show you all the cases.