This error occurs when the default tracking link domain in Marketo has been removed and not replaced with a properly configured Branding Domain CNAME. The tracking link domain cannot be restored to the default once a Branding Domain has been put into place, so you would need to either restore the old Branding Domain or configure a new Branding Domain CNAME and enter that into Admin > Email.
If you have a record of the original Branding Domain, you can use it to look up the default tracking link domain using a tool such as MXToolbox. You can look up your old CNAME and it will show you the tracking link domain it points to, which should look something like "mkto-ab01234.com." You could then follow the steps here to set up your new Branding Domain CNAME for your tracking links. Alternately, if your old CNAME record is still on your DNS, you could have your IT department look it up.
If you do not have a record of the original Branding Domain CNAME, you can contact Marketo support who can look up your original tracking link domain on our internal tools so you can set up a replacement CNAME.