This is because your disk format is MBR (Master Boot Record) an dit supports only 4 primary partiitons. You can check it from Windows Disk management tool by
Right Click Disk 0 on the left -> select Properties -> In that Volume Tab. You can see either Master Boor Record or Guided Partiiton Table (GPT). If it is GPT, which is a new disk partition method can support numerous primary partitions. But MBR is old and can support upto 4 primary partitions only.
So you have to use one primary partition to create a logical or extended partition. Inside that you can install Kubuntu. Never format or delete that newly created primary partition as it removes the whole logical partition too.
There is another option, to wipe whole hard disk and reinstall everything in GPT mode. I have seen tutorials to convert from MBR to GPT without reinstallation, but I didn't try it yet. Do it at your own risk