Ensure that your phone and browser have the permission to download and install apps from 'unknown sources' or download and install unknown apps'.  For this, you might need to: 

  •    Go into phone settings, and into your apps to find the browser app you're using 

  •    Look in app permissions or advanced settings 

  • ●   Allow installation of 'unknown apps' or apps from 'unknown sources' 

*Please note that the screenshots below are only indicative and there might be minor variations in the settings and permissions options in different smartphone brands and models.