12/10 Tuplet

Tuplets with a total duration that cannot be represented with a single note value are tricky but possible. The way to do this would be to first create an empty measure of the desired length, then use its full measure rest as the basis for the tuplet. Then copy and paste to where you actually want.

Oops, Marc beat me to it. But, yes, create a temporary 12/16 measure and convert its whole-bar rest to a 12/8 tuplet and then copy and paste that to where you require it.