AngularFire2 - Cannot find module @firebase/database

You could try with:

$ rm -rf node_modules/
$ npm install
$ npm install angularfire2@latest --save

or to change AngularFireDatabaseModule by AngularFireDatabase.

I think it has to do with an issue with npm. When using yarn to install the modules, everything works flawlessly.

yarn add angularfire2 firebase

tldr: Node: 8.4.0/npm: 5.2.0 has issues, yarn works