It looks like the Bitrise build is failing because it's trying to use the 11.4 simulator but only iOS 12.0-12.4 simulators are available on the currently selected stack.
It should be a fairly quick change to update the iOS step to use a 12.x simulator, but it can only be done by the owner.
Here's the smallest change to the bitrise.yml file that I think should fix it. It just removes the iOS step truing to use an 11.x simulator. Though you may want to look into using a newer simulator version as well.
It looks like the Bitrise build is failing because it's trying to use the 11.4 simulator but only iOS 12.0-12.4 simulators are available on the currently selected stack.
https://github.com/bitrise-io/bitrise.io/blob/master/system_reports/osx-xcode-10.3.x.log#L1077-L1080
It should be a fairly quick change to update the iOS step to use a 12.x simulator, but it can only be done by the owner.
Here's the smallest change to the bitrise.yml file that I think should fix it. It just removes the iOS step truing to use an 11.x simulator. Though you may want to look into using a newer simulator version as well.