Had a similar issue while installing "Lua" in OS X using homebrew. I guess it could be useful for other users facing similar issue in homebrew.
On running the command:
$ brew install lua
The command returned an error:
Error: /usr/local/opt/lua is not a valid keg
(in general the error can be of /usr/local/opt/ is not a valid keg
FIXED it by deleting the file/directory it is referring to, i.e., deleting the "/usr/local/opt/lua" file.
root-user # rm -rf /usr/local/opt/lua
And then running the brew install command returned success.