cant reinstall osx

I would try and reformat the drive (using HFS+) through disk utility and see if that works. Sometimes a complete reformat will do the trick. I am assuming that you are using the disk that came with the system.

 

Yes, it does matter as if your MBP never came with SL then it will be hard to get it installed (unless it is a MBP that was once offered with it, such as early 2011). Your responses are vague, but if the version of SL you are installing is lower than the one included with the MBP originally it is not going to happen (for example if it is an early 2011 and you are trying to install a retail SL it won't work as the retail versions are lower versions than were included when the MBP was originally released). Without more information it is hard to help. We need MBP version and SL version to help further.