First, did you state specifically the information you want your buyers to fill when placing an order? If you did this and it’s part of your gig description, if the buyer doesn’t fill them, the order count down will not start. If this is the case, there is nothing to fear and you can wait as long as you want for the buyer to come back and fill that.
If it is the case where there is no fixed requirements for the gig and the buyer has placed an order whilst the order count down has begun and buyer didn’t provide you with order requirements, If the order count down will take a while, I’ll suggest you wait a bit for the buyer to come back online for that because cancellations affect your rating and even if you want the buyer to cancel, he still has 2 days to accept the cancellation.
If any of these do not apply to you, you can cancel outrightly.
All the best