TLDR; => Try again from the EOS live environment!
You may have forgotten, but you had a similar question some time ago. ==>
The answer still applies - just leave out the “LVM” stuff.
Gparted should be able to do this for your simple ext4 in luks case. (Never tried this myself though; always do it via cli).
If not, try doing it manually. You can see the required commands in your error details.
If this still doesn’t work I could check your commands and/or provide the needed information.
But, in any case, do this from the EOS live environment (see below).
I think you misunderstood @Hystrix .
Of course you can shrink or grow luks partitions! You just need to make sure that the luks partitions filesystem isn’t mounted (“open”) during certain commands. It’s just advisable and sometimes required to shrink/grow from another environment, e. g. the EOS live environment.