Take a look at the top right of your editor, you can see the option to adjust snap divisor.
Now with that in mind, the "rare snap divisor" notice usually happens when your object snaps to anything other than the usual straight beat (1/2, 1/4, 1/8) or swing beat (1/3, 1/6).
This doesn't mean the map is entirely unrankable though, because there exist songs using irregular tuplets. Only if the song doesn't have such rhythm but the thing still pops up that you really should take a look at the map, most notably, your timing.