If you want to install the STNDR Android app, you need to activate the option "Unknown sources" first. Only this way you can install apps outside of the google play store.
First open the app-menu.
Now open the settings.
Navigate to "lock screen and security"
Activate the switch for "Unknown sources".
Navigate to the downloaded STNDR apk via "Downloads" App, "My Files" or similar.
Now just click and install.