Cancelling an incomplete order will affect your order completion rate.
Not cancelling it will just make it stay there forever (or until Fiverr decides to do something about them)
So, assuming you’ve already used the ‘Nudge’ button on the order page, I would advise contacting the buyer through the Fiverr Inbox and remind them that they’ve placed an order with you but haven’t provided you the information you need to start working on it.
Once you tell them that, you can even ask the buyer about the info you need so that you can then manually start the order yourself.
IF, however, you’ve contacted the buyer and they didn’t respond, they might have their perfectly valid reasons (think unforeseen life happenings), in which case you can simply leave the order as is and move on to the next buyer - leaving it as is and forgetting about it will not affect your metrics.
P.S. you’ll be amazed how many buyers order and don’t leave any info thinking that they already sent the info when in fact they didn’t, at least not in the right place