If the space you want to partition is part of an existing drive, you'll see Unallocatednext to existing partitions on that drive.
Ọ bụrụ na ohere ịchọrọ ịkewa bụ akụkụ nke ụgbọala dị ugbu a, ị ga-ahụ Unallocated na- esote akụkụ nke dị na ụgbọala ahụ.