I am engaged in a rousing battle with the Arceans.  As shown in the screen snapshot below, I was trying to use a missle attack agains the Arcean fleet of 57 ships when I got the message "Cannot be engaged in battle".  Given that I was not trying to marry them, I am assuming that the message means that I cannot attack them.  Why not?  Is it because the game thinks that I am trying to attack the exclamation point icon instead of the fleet that is underneath it?

Here is a save file that you can use to try to attack yourself:

https://www.dropbox.com/scl/fi/0swfzyim7dktl685wut2g/Cannot-Engage-Under-1.60.457293.GC4Sav?rlkey=g5khspbt1ijc7rgl0aec9avbo&dl=0

Just load the saved game then go to the planet named Misam V and you should see what is in the screen snapshot below.