To enable Direction API use for your Google Map API key do follow:

1. Login to Google account,  Open your project on https://console.cloud.google.com/apis/dashboard, here: http://prntscr.com/sb1ng0

2. Type Direction API and click follow: http://prntscr.com/sb1nwo

3. Click Enable button, when Enabled, you will see: http://prntscr.com/sb1o3g

4. Go to Your API key, edit and add Direction API to here: http://prntscr.com/sb1ou0

Done.