"This is a startup partition. You can't erase it" On Disk Utility I want to expand my Windows partition. I saw all the tutorials on youtube and they said to drag the drive on the corner. However, the Disk partitions doesn't have a drive for me to drag and said that it was a startup partition so I can't decrease it's size.
In order to increase your Windows partition, you will need to first free up disk space on your Macintosh HD partition, as it appears to be full.  128GB isn't very much these days, I would recommend getting a larger hard drive, but transferring larger, less frequently used items such as movies or videos can often help free up large amount of space.