Still getting "This domain is not registered with Tiny Cloud" message even after importing the API key

I am using the example TinyMCE code that's available on their docs:

<editor [init]="{
  apikey: '<secret>',
  height: 500,
  plugins: [
     'a11ychecker','advlist','advcode','advtable','autolink','checklist','export',
    'lists','link','image','charmap','preview','anchor','searchreplace','visualblocks',
    'powerpaste','fullscreen','formatpainter','insertdatetime','media','table','help','wordcount'
  ],
  toolbar:
   'undo redo | casechange blocks | bold italic backcolor | \
   alignleft aligncenter alignright alignjustify | \
   bullist numlist checklist outdent indent | removeformat | a11ycheck code table help'
}" initialValue="Welcome to TinyMCE Angular"></editor>

Except I added the apiKey: stuff in because I am getting a message that says This domain is not registered with Tiny Cloud. I have made an account and I have checked to make sure the API key is correct.

The "This domain is not registered with Tiny Cloud" error comes in two flavors:

1. "This domain is not registered with Tiny Cloud. Please see the quick start guide or create an account."
2. "This domain is not registered with Tiny Cloud. Please review your approved domains."

Steps for troubleshooting both errors are described in the documentation: 1, 2.

You surely have an account, and the API key is probably correct. But you may have not set up the "Approved Domains" section in your Tiny account or you may have issues with the referrer header.