This message is displayed to the users if their account (contact profile) is not verified/activated. 

In such a case, please go to Customers > Contacts, click on the contact that is seeing the error message and hit 'Send activation email' button on the contact details page. The customer can then use the link sent via email to set up a password and log in.

As an Admin, you'll also be able to set up a password for the contact using the 'Change password' button on the same page.

To do this for multiple unverified contacts, please click on the hamburger menu on the Contacts page, click on 'Unverified contacts' to retrieve the complete list of unverified contacts:

You can bulk select the desired contacts from this list and click on 'Send activation email'