Splitting surfaces problem

Hello,

I'm building a car for my final graduation project. The car is entirely built in Solidworks as i'm not too experienced with Solidworks. I know things, but not all of them.

I already splitted some as you can see in the picture:

So I tried splitting the other side as I want a car with windows on both sides, but then it said that I couldn't split: https://www.dropbox.com/s/3sa85fpqs5couz4/cant%20do%20it.png

So I saved the file and reopened it. When open the file it says: Part contains unsupported body operations. Featureworks could not proceed further.

Okay well cool... but I couldn't split anymore faces.

Here is the file: https://www.dropbox.com/s/vn0wh7m7a87g9tk/test%20finish.SLDPRT

SolidworksSketching