Thanks for pointing out the space/comma mixup. I edited the post to correct it. Since it is still useful after 4 years, I'll update the status of FT support in current releases. It is much easier now.
OpenWrt 21.06 and earlier are EOL now, so you should not be using it by now.
If you set option key (should always be the case with LuCI): Starting with OpenWrt 22.03, when not using WPA3, all you need to do to make FT work is to enable 802.11r from LuCI (option ieee80211r '1' in /etc/config/wireless ). With WPA3, including mixed mode, you must disable "Generate PMK locally" in LuCI (option ft_psk_generate_local '0' in /etc/config/wireless). There's no need to touch keys, mobility domain or anything else.
With current snapshots, (and future major releases), the "Generate PMK locally option" will be automatically disabled when using WPA3, and will not even show up in LuCI. It does not mean that if there is an option ft_psk_generate_local '1' in /etc/config/wireless it will be ignored! If it is set to 1, then it will break FT with WPA3/SAE.
For completeness: if you use the wpa_psk_file option (a PSK file) instead of key, then r0kh and r1kh need to be set manually.