This definitely isn't something I know about but googling suggests that you do exactly the same with your device as you would with an Android phone. That means that you'll first need to go into the device's security settings to authorise accepting content from unknown sources. That's explained here (together with some suggestions for alternative app stores):
https://www.digitaltrends.com/mobile/android-app-stores/
If that doesn't help (and nobody else replies here) I suggest registering on this dedicated forum and asking your question there:
http://www.minixforum.com/forums/neo-x8-h-x8-plus-x8.18/