Firstly, and most importantly, executing grub-install requires super-user privileges. Rather than adding it to PATH you should instead run the command through sudo. A regular user typically does not need any of sbin directories in their PATH.

As for the command itself, there are three things wrong with it:

1. PATH should only include directories whereas you tried to add to it a path to an executable. So rather than /usr/sbin/grub-install/grub-install you should just add /usr/sbin.
2. White space is significant, so the space before colon would make your command not work anyway.
3. Rather than appending to PATH you’ve overwritten the variable. Instead you need PATH=$PATH:/usr/sbin/:/usr/local/sbin (notice $PATH: at the beginning of the assignment).

Also, export is unnecessary since PATH is already an environment variable. (That’s also bashism but that’s likely an irrelevant issue).