Windows can support BTRFS with an unofficial driver:
https://github.com/maharmstone/btrfs
I’ve used it without issues, so world recommend it as long as you’re aware of the disclaimer:
You use this software at your own risk. I take no responsibility for any damage it may do to your filesystem. It ought to be suitable for day-to-day use, but make sure you take backups anyway.
I’ve not used the fedora installer, but from what I can tell you could try formatting sda6 with the filesystem you want and trying the installer again or maybe try clicking the + in the installer and see if it gives you the option to format sda6 and give it a mount point