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.