Chartreuse doesn't really have any good substitutes that I can think of, it's got a very distinct flavour that's nothing like absinthe (I like to lovingly describe it as lawn steeped in everclear).

Still make the drink with absinthe and see how it goes though - it'll taste totally different but it might still be good, and the ABV and (lack of) sweetness of chartreuse and absinthe are at least fairly close.

Genepy is the best substitute. But now that the monks aren't making as much, other subs have popped up.

But definitely not absinthe.