In order to watch live sports channels, movies by different iptv apk, we usually need to install apps on our tv box. If you use Firestick, need to set it to support unknow sources apps first, then could install freely.
First, click Setting on the main page.
In Settings select My Fire TV (if you are still on an older version it may say Devices)
Click Developer Options
Choose Apps from Unknown Sources and click Turn On.
Then the firestick wont stop unknown sources apps installation.