When I buy the ice habitat , the game wanted that I invite 3 persons to open the habitat but I don't have any friends in the game . There's an alternative way but for this I need to pay 6 gems and I was saving gems for other things . What should I do ?

Hi and welcome to forum :)
Maybe times to add friends ;) . we have this sticky for that
https://forums.socialpointgames.com/topic/88/official-new-neighbor-find-add-thread

Done . Now it's time to wait until some friends come ...

And you can add yours so others can add you too ;)