Did you use play.date/account/sideload? For some reason I get a server error with this PDX in particular using that method.
It should work if you unzip the zip file and either
- Put your playdate in data disk mode and copy the entire .PDX folder into the games folder of your playdate, OR
- Use the Playdate simulator with your playdate connected to your computer, choose the option from the device menu to transfer the game to device.