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
One example: Butts
52.4054875, -1.5226096
https://maps.app.goo.gl/vGWxfASV2E5HheBR8