I’ve always had trouble getting into single mode in cloudlinux. Trying to type single would show up as gibberish. Looking at a server today, I accidentally came across a way around this, by hitting c (to go to the grub command line) and esc (to exit back). Once done I could use append (a). Previously append or edit did not work.
At the GRUB splash screen at boot time, press any key to enter the GRUB interactive menu.
type c (goes to grub command line) then hit esc
Select Cloud Linux with the version of the kernel that you wish to boot and type a to append the line.
type single and hit enter to boot