Why do I get an error message when trying to install apps?

[Trey Scales]

app not installed error message please help

I have a note 5 and have realized that you cannot sync your contacts with the preloaded version of facebook that comes on the phone so I was trying to uninstall updates for facebook and install an earlier version of facebook from apk mirror that does allow contact syncing and everytime I try to do this I get a message that says app not installed. I was wondering if I was doing something wrong or if it is not possible to do this. Please help.

 

[B. Diddy]

Re: app not installed error message please help

Welcome to back! You have Unknown Sources checked on in Settings>Security, right? And you're tapping the apk to install it?