App Not getting installed

My application user is not able to install an application on his Samsung A9 pro.
But when i change the package name of the application signed with the same keystore as former, he is able to install on the same device.

What is restricting the application to get installed? is it something to do with caching?
How to resolve the issue. Please suggest.

= Can you provide us with more details of the error. Also i would like to know was there any recent software changes done on your smartphone