What does "error validating access token" mean?

The short answer: your Digital Dashboard and Facebook are not connected and cannot communicate. 

Go to "Connect/Reconnect your Facebook Page"

A much longer answer, that I'll breakdown below, is either that you have not connected your Facebook to begin with OR you may have changed your Facebook login information. If you change your Facebook password or change the email address that you use to login to your Facebook account, this will disrupt the line of communication between your Dashboard and Facebook.