You are trying to write within your program files folder. Possibly you don't have admin rights and therefor are not allowed to write files there. Always first set your Current Folder in Matlab to a folder you have access and permission to. Files you write will end up in that folder. Try changing your current folder to a folder you do have access to.
If you want to write your file to a specific other folder, you can also use the full image address in imwrite() - just as you used the full address in imread()
