The reason why Windows won’t let you shrink the volume is as the message shown in Disk Management suggested, because there are immovable system files at the very end of the volume, as this screenshot from utility shows us. there are multiple things you could try to work this around.
1. Run the Disk Cleanup Wizard, making sure to remove the hibernation file and all restore points.
2. Disable System Restore
3. Disable the pagefile ( Open up System in Control Panel, then Advanced System Settings \ Advanced \ Performance \ Advanced \ Change \ No Paging File.
4. Disable kernel memory dump. In the same Advanced Settings, go to Startup and Recovery \ Settings and then change the Write debugging information drop-down to “None” to disable the kernel memory dump.
5. Disable Hibernation mode in your power options \ advanced power options screen.
Reboot the machine, and then delete your c:\pagefile.sys file, following these instructions if you are having issues.
details about the fix and cause of this problem, see