Got this error

[meetmec12]

Since you're using the custom project type, you must provide the ionic:serve npm script so the Ionic CLI can
serve your project.

[xfq]

Making the following modifications to package.json solves the issue for me:

   "scripts": {
+    "ionic:build": "vue-cli-service build",
+    "ionic:serve": "vue-cli-service serve",
     "serve": "vue-cli-service serve",
     "build": "vue-cli-service build",
     "lint": "vue-cli-service lint"
   },

(Reference)