So, for Angular 1 apps you need to explicitly define the application and controllers, and they are different form one another. This may read differently from examples that you've seen for NG2+.
var myApp = angular.module('myApp');
myApp.controller('mainctrl',['$scope', function($scope){
$scope.message='Hello Angular!';
}]);
The controller definition includes the dependency injection array used by NG1, and you'll notice that the name to use for the controller in the HTML is a string, not a variable.
Then amend the html declaration tag:
<html ng-app="myApp">