for how to boot into single user mode. This gives you a root level terminal. From there you can run visudo and/or add openhab to the sudoers group or fix whatever you need to at the root level. Then remove the changes to cmdline.txt in the boot partition and reboot and you should be fixed.
However, this sounds a whole lot like something went wrong with the install and I’m not sure I would trust that something else isn’t wrong. If it were me, I’d reinstall.