The code you are currently using is to the Google Domains api. The error means that the user you have currently authenticated with doesnt have access to the domains account. Make sure to grant the user access and they will be able to post to the Domains google+ page authentication
Note:
There is a difference between the Google+ API (Socialmedia website) and the Google Domains API (Gsuite)
The google+ api is read only and does not allow for programiticly inserting posts into Google+.
How authentication works
When you authenticate your application using these scopes
$this->client->addScope('googleapis.com/auth/plus.me');
$this->client->addScope('googleapis.com/auth/plus.stream.write');
$this->client->addScope('googleapis.com/auth/plus.stream.read');
$this->client->addScope('googleapis.com/auth/plus.pages.manage');
$this->client->addScope('googleapis.com/auth/plus.media.readwrite');
You are asking the user can I do these things on your behalf. However if the user does not have permission to do something your not going to be able to do it.
calls to this method must be made by or on behalf of a Google+ Page.
The user you are authenticating with does not have access to a business page. So your application cant write to a business page. You can only post to a domain account if you have a gsuite account. If you don't have one then you cant post to it. you cant just give them the id of your business page because again they dont have access to write to your business page.