This plugin is likely not whitelisted in Construct build server, so you might need to build with Cordova CLI.
1. Export your game in Construct 3 as iOS Cordova project (not as XCode project).
2. Edit config.xml file to include "cordova-plugin-hide-home-indicator" plugin.
3. Build with Cordova CLI:
construct.net/en/tutorials/building-mobile-apps-locally-21
There are detailed and up-to-date tutorials in the Internet about how to build with Cordova for iOS, try googling.