Can not install RHEL 7 on disk with existing paritions. Menus for old paritions are grayed out. Can not select mount points for existing paritions. Can not delete them and create new ones. Menus say I can but installer says there is a partition error. It shows me a complicated menu that warns be what is about to happen and asks me to accept the changes. It does not seem to accept "accept" as an answer. Booting to the recovery mode on the USB drive and manually deleting all paritions is not a practical solution.
You have to assume that someone who is doing manual partitioning knows what he is doing. Provide a simple tool during install so that the drive can be fully paritioned and formatted. Do not try so hard to make tools that protect use from our own mistakes. In the case of the parition tool, it is too complicated for a common user to use so don't assume that the persion using it is an idiot. The common user does know hardware and doesn't want to know it. He expects security questions during the install and everything else should be defaults. A competent operator will backup anything before doing a new OS install. He needs the details and doesn't need an installer that tells him he can't do what he has done for 20 years.