You might have created a too small system partition or VHD when you installed or upgraded to Windows 10.
With older Windows editions, this was no problem, you could simply extend the partition via Disk Management GUI if space was available on the disk.
With current Windows versions, there is a recovery partition appended at the end (right side). This makes it impossible to extend the system partition with on-board GUI utilities.
Solution / Workaround
You can either use a third-party tool like MiniTool (Windows Software) or Gparted (Linux Boot Disc or ISO) to move the recovery partition to another destination on the disk, so you can resize your main partition.
As an alternative, if you are absolutely sure you do not need the recovery partition, use Windows’ DiskPart utility using these commands to delete it:
sel dis 0
sel par 3
del par override
Afterwards, you can extend your system partition via Disk Management.
Note: The script above assumes you have exactly one HDD in your system, using the default partitioning schema.