Sorry, you can't. You can't make assets specifically for the game jam. From what I know, you can use assets from other games you have made, before the jam, and you can definitely use asset packs.
"You may use pre-made, publicly available, and purchased assets,"
I got this from the rules page of the game jam. From this, and everything else I read, I guess you can make assets for the game jam, just not any code. I could still be wrong though, so maybe try asking one of the moderators of the jam?