The message "Linkedin account is missing for one or more nodes in the campaign." indicates that certain steps in your campaign, which utilize LinkedIn, lack an associated LinkedIn account. Ensure that you've chosen a LinkedIn account for each relevant step in your campaign.
Here is how to fix this issue :
Go to your campaigns
Open the target campaign
Look for steps LinkedIn visits, LinkedIn messages, LinkedIn invites or Conditions.
Make sure that for each of those steps there is a LinkedIn account properly selected.
For exemple this is not correct :
And this is correct :
Do not hesitate to contact us if you have any issue.