Here is a screenshot of just what is happening. (Nothing changes when I click the "OK", I just thought that may be relevant")
But here is an image showing that I have plenty of space on my hard drive.
I had a previous partition on bootcamp, but removed it because it had somehow used up 39of the 45 GB I gave it, despite practically downloading nothing in the Windows partition.
I am using a USB, which I believe is necessary for this process.
Thanks for your help (System info)