Hi - I have setup a campaign in SFDC and in Marketo.
Marketo is not recognising the campaign even through the campaign is coming up as an option in the field - underlined in red below:
I have seen this happen before. Sometimes the only thing to be done is to delete that flow step and then try adding it again. That sometimes gives Marketo its much-needed Cher from Moonstruck moment (aka "Snap out of it!" )
Another thing that can work (as long as you selected the campaign from the dropdown and it still exists), is simply clicking on that box and then clicking off of it. That will usually get rid of the squiggle for me.
or place your cursor in the "evaluator" field (IN) and cycle through the fields using the tab key. In other words, there's no one way to resolve this maddening issue. It's one of those things that we've all learned to live with as Marketo experts.