Michael,

I got that error several times when installing an add on campaign.

What's happening ?
One unit or way point of the mission file you refer to has got the same number as an airbase (Deblin) in your airbases.dat file. The airbases.dat has priority over the mission file apparently and CFS2 believes your destroyer is in formation with Deblin. Thus the error.

The easy fix
To exchange your customized airbases.dat with a fresh one in which you put the entries required by the He111 campaign.
Drawback : you have to manage several airbases.dat files, and change from one mission to another.

The hard fix
Open your airbases.dat file and find the number of the Deblin airbase.
Open your mission file and search for that number. You'll certainly find that a ship supposed to lead a formation with your destroyer is using this number. Change it to another unused number (unused means by the mission file and by the airbases.dat), usually below 8000 is safer. And change every occurence of it in the mission file.
You then have a mission file compatible with your poland and He 111 campaigns.

Guy