For Angular developer:
When I need to use agm-direction package, the console shows that "you have to have a credential key first, please go to here", but I already have one so I can view the google map.
Aftet a while, I found the only thing you need to do is go to Direction API and enable it, then wait for about 10s, you are good to go. The whole thing sums up that the console log didn't tell what API is needed exactly.