Hiya - I was getting this too. Making a new site with bootstrap automatically installs a read only bootstrap.css in the css folder. So when you get the prompt to save that first css file I just named it bootstrap2.css You don't end up using it but it lets you progress through the rest of the lesson. I tried saving it as styles.css to use later in the tutorial but it won't file in the right place (i.e.in the css folder - the file seems to disappear) so I just used the bootstrap2 method as a workaround. I'm sure there's a more correct way to proceed but this at least lets you go forward anyway. They must have changed something in the set up protocols between making the tutorial and the release.
Hope that helps. Seemed to work for me.
Is there any chance that any of you were in the prerelease program? Earlier builds of DW had this issue, but it had been fixed by the time the production version was released. If you did have a prerelease version, there would be some clean up to do since it could have left some old files from that version. If that's not the case, I'll keep digging. If you were not in the prerelease program working with early builds, could you let me know which OS you are using?
In the meantime, Gail's suggestion is a good workaround.
Also, I assume you have the latest version of DW available from Creative Cloud? If not, it's a good idea to update if you can.
Just to let you know I was using both tutorial files and DW2015 downloaded in the last few days and I was getting this problem.
I just wanted to let you know that I haven't forgotten about this. I'm working with the product team and it looks like there is still a scenario where this could occur. I'm just trying get the details on what that scenario is. I'll update ASAP.