There should not be a space between the entries in $PATH, just :. Plus, this is syntactically wrong: export allows to set multiple variables separated by space, like export A=a B=b. You’re essentially doing export A=a B which makes it ignore the B part as it is nonsensical.

Also path should contain directories, and /usr/sbin/grub-install/grub-install isn’t a directory. In fact it almost certainly does not exist at all.

Path should contain at least /usr/bin. That’s why no command works, they’re all in /usr/bin. The shell looks through all the directories in $PATH (separated by :) to find commands.