I've moved this thread to MV Support. Please be sure to post your threads in the correct forum next time. Thank you.
This is likely a setting on whatever map you're on when the enemies are encountered. How did you create your map - did you use one of the sample maps? Did you copy it from someone else's project? Those are the only ways I can even think that would result in you having a map with battlebacks specified that aren't in the project folder - you would not have been able to enter them yourself if they weren't there to start with.
So, check the map properties, and change the battleback to something that IS there. Or grab the SF battlebacks from the DLC folder if they're there, and copy them into your project.