Look into this topic - https://bbs.archlinux.org/viewtopic.php?id=267383
I’m on Arch and not sure if this will work for you.
I fixed this problem by editing this file
sudo nano /etc/grub.d/proxifiedScripts/linux
in line 204 where this error occurs I deleted it and pasted the line from this topic link above
I just pasted this line instead of line 204
changed this >
linux=`version_find_latest $list`
into this >
linux=`echo $list | tr ' ' '\n' | sort -V | head -1 | cat`
now grub-customizer runs perfectly for me and grub-mkconfig runs without any errors