Do yarn add firebase-admin

package.json

@@ -13,6 +13,7 @@
     "body-parser": "^1.18.2",
     "dotenv": "^4.0.0",
     "express": "^4.16.2",
+    "firebase-admin": "^5.5.1",
     "mongoose": "^4.13.2"
   },
   "devDependencies": {