1 - Invalid code - Error in the Facebook App.

Make sure you have entered the Facebook Client Secret in the Site Setting.- this is a required one

2 - Error: Error registering phone number

If you open your storage/log/laravel.log file, you will see the actual error. But in most cases can be if you don’t have all the “Permissions” approved from Facebook.

array (
  'error' => 
  array (
    'message' => '(#200) You do not have permission to access this field.',
    'type' => 'OAuthException',
    'code' => 200,
    'fbtrace_id' => '-------',
  ),
)

3 - This Access token can’t be decrypted.

Make sure you use latest version of the module, and that your client has reconnected to WhatsApp.