Don't have rapid growth potion.

I'm supposed to have rapid growth potion from the gnome to grow the sapling but for some reason i don't have it in my inventory...

Are you playing this game on the PC or iPad? There are a lot of glitches with the latter. If you got the potion from the gnome, it should be there! The only other solution is to uninstall/reinstall and start the game over when things poof from inventory.