This issue can sometimes be caused because of multiple constituents sharing the same email address. Perform a search in Altru to make sure that the email on file for the constituent is unique to that constituent's record.  If you find that email on another record, if it is the same record, merge the two constituents together.  If they are separate records, delete the email from one of the records.

This issue can also occur if the Constituent has received a Membership Renewal Email containing a Membership Renewal link. The Membership Renewal Email will prompt the Online Info Tab to appear on the constituent's record, but since these constituents have not registered for web forms they have no data to populate this tab. These constituents should be able to register for web forms and resolve the error message on their Online Info Tab.