You need an OS image that came out after the release of the product, or you can upgrade the SD card in your Pi 2 and that should do the trick, provided the OS version (e.g. Stretch) came out since the 3B+. Raspbian Buster came out recently so you probably just want to use that, knowing that it supports all models.
The answer to the question "What are the ...
The most likely explanation is the boot files need updating.
You should be able to this on the Pi2 by doing a (sudo) apt update and apt upgrade.
If that doesn't work you might have to do a sudo rpi-update with the SD card in the Pi2.
Buy a new SDCard that's 16GB or 32GB and never worry about available space again.
Also ditch NOOBS, it wastes 1.3GB of space to keep the recovery system you'll never use. If you need NOOBS use NOOBS Lite or PINN.
Make sure your device has file system over entire sd card with raspi-config command. Also, try commands such as sudo apt-get autoremove and sudo apt-get clean in order to remove unnecessary dependencies and cache. Also, if you're using 8 GB card I'd suggest using Lite version of Raspbian, since this is a very small ammount.
Note: The question asks about Minecraft Pi, which apparently isn't just Minecraft on a Raspberry Pi.
Minecraft must be told to /save or /quit so that it can properly shutdown and save. If you do not properly save the changes to disk, it is unlikely you will be able to recover your world or the changes you made.
Using the Minecraft console, you can issue /...