When trying to manually add a Salesforce Contact into a Pardot campaign, error code pops up. Why?

Under the details section under certain contacts, there is the section "Prospects lists" and it usually lets me add contacts to Pardot campaigns manually through there, but for some contacts this message shows up "Prospect could not be found matching fid: '0034x000001d1obAAA'". What does this mean and how can I fix this?

#Campaigns  #Pardot B2b Marketing Automation  #Pardot  #Campaign  #Error Message