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

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

2. Search within app permissions or the advanced settings section 

3. Allow installation

*Please be aware that the screenshots provided below serve as general guides only. There can be slight differences in the settings and permission options across various smartphone brands and models.