Try a Different Browser
The default Android browser on certain phones (such as the Samsung Galaxy S4 Active) will fail to install apks downloaded from the NEON website. To workaround, open the download website with another File Explorer or browser (such as Chrome). the direct link to the Android download page is: https://neon.trxsystems.com/downloads
Verify Android Version
Verify that you are running KitKat (4.4.4 / Android API Level 19) or later. If not, you will need to obtain a phone that runs the supported Android version.