Too many partitions.
You can have up to four primary partitions on your basic disk. You already have four.
What you can do is delete one of your primary partitions and create a new extended partition in the empty space. You can set up as many logical drives within your extended partition as you like.
Of course, if you want to just use that unallocated spec for the c drive, you could right click on c and choose extend volume.
I'm assuming you don't want to convert your disk to dynamic, since you didn't choose that in the vid.
