Was able to sideload!! But...bAnother error, this time on the PlayDate. 
Viewing post in [PlayDate] Fishing Simulator comments
Same error for me as well.
Not sure if this is the problem you are having, but I was having this same issue sideloading my zipped game as well.
I was using the windows built in zipping utility and it wouldn't sideload properly.
I re-zipped it with 7zip and it sideloaded normally, not sure why but it fixed the issue for my game.