The card reads "Choose a Space Marine that has at least 1 Support Token (if able). Discard 1 Support Token from him and 1 Genestealer engaged with him (of your choice)"
If I apply the effect strictly, even if there are no marines engaged with GS, one of the marines MUST lose (=waste) a support token.
However my guess is the card is badly worded and should be "choose a marine with a support token AND engaged with at least one GS… (the rest the same)"
This will fit the intention and theme of the card, and also will balance the fact that the card has a supposedly "benign" effect
What do you think?