Is super easy to find out what can be wrong... put the bike upside down.... have a friend. wife or who ever to shift the bike up and down and see how the RD moves or if the RD wheelies aren't aligned with the cassette or freewheels cogs.

Check the cable tension as well, pretty easy... just touch it, the cable has to be tensioned, if lose like a spaghetti then is clearly that the problem goes that way.

If the Tension is ok and the RD is not aligned with the cogs then the fix is pretty easy.. just ask here how to fix that.