This is a reference post for those who have trouble playing android. It includes step by step methods on how to fix your issues.
If these don't work, I recommend playing on PC if you're able.

How To Fix Invalid APK

This error happens for one specific reason - android security settings. However, there could be multiple issues at play.

• Files over 2 GB in size will automatically be denied. The only way to fix this is to use Joi Play to run the game.
• If your file is less than 2GB and you still receive the invalid apk error, then you will need to go into your android phone settings and change the "install unknown apps" settings. Below is a step by step screenshot of the process (from left to right) once you enter the settings menu.

In the case of HPC, you will need to do both of these things, most likely.

Q:

"Why don't you just sign the file?"

A:

"Tried. It's more technical than you'd think if you're trying to "hack" it and I haven't downloaded the android development suite to do it the proper way. Even then, it will take time to learn how to do it - and time is not something that I currently have. In other words, this is very low on my list of priorities - especially when you consider that I always intended for HPC to be a pc game anyway. Eventually , I'll get around to it - when I have 3-4 hours to spare."