There are no free mbr slots on your disk

You can only create 4 primary partitions on an MBR hard disk. If you need more then you need to create an extended partition (as one of the 4) and make additional logical drives within that partition. Any good disk partitioning software will let you convert one of your primary partitions to an extended one (not your C: drive!). (I use Paragon Disk Manager because I got it free on a cover disk, but there are many similar programs).