240 volt 20amp outlet
A NEMA 6-20 does not require a neutral. The devil is in the details. A NAMA 6-20 configuration is for a 240V, single voltage connection. If it were to have a neutral the configuration would be a dual voltage NEMA 14-20, 20A 120/240V.

If you don't need the neutral you can simply wire nut it off and leave it in the box.