I also faced similar issue earlier.
I was unable to extend or make small my any partition with Windows' in built 'Disk Management'.
Then, I used 'AOMEI Partition Assistant' on someone's suggestion and it worked without errors, but it do suggest to backup your files, otherwise if anything wrong happens you may lose all your data.
As @harrymc suggested you may also use 'EaseUS Partion MasterFree'. By the way, you can use any reliable 3rd party software.
As you are trying to do operations on system partition, I would suggest you to make a Bootable Media in case your partition get corrupted. (AOMEI Free edition have this feature)
For me extend operation on system partion didn't cause any errors, but it depends on system to system.