Because MobileBuilder is now yet hosted on Google Play Store, before you can install it you need to enable "Unknown Sources".
The exact names of the settings entries listed below may vary slightly from device to device, but the overall process should be very similar.
1. Find your phone's main settings menu, then look for an entry titled either "Security" or "Lock screen and security".
2. From this menu, simply tick the box or toggle the switch next to the "Unknown sources" entry.
3. Press "OK" on the popup
For Android Oreo (8) Google has removed that Unknown Sources switch and turned it into an “Install unknown apps” permission.