Did you allow installations from unknown sources? You can enable this here:
Settings > Security > Unknown Sources
If that doesn't help you could try to disable Google Play Protect if it's enabled.
Open "Play Store" > Menu button > "Play Protect" > Disable "Scan device for security threats".
Also, did you rename the apk or execute it from an SD-card?
I've also built a arm64 apk for another user with install problems, maybe you could give this version a try: https://mega.nz/#!8PAxhYpJ!7RDJaLiLEA5592t_PUBplFp2BG126dn2AG3ejGkG4fY