I found this isue with my game too and spent ages trying loads of different things. the first thing is to try a different zip program. i found windows built in zip didnt work but winrar zip option did.i tried just rezipping ur game and that didint woirk, it needs more info in the pdx . should read something like this then it seems to work :)
"
name=Pop pop
author=therussianbeargame
description=Pop pop
bundleID=com.therussianbeargame.Poppop
imagePath=images/
pdxversion=11000
buildtime=707837117"
Also with this if you then place a 350 x 155 px image called "card.png" in the images folder it will show up on the playdate menu