Start with brew doctor
which will show you errors with your brew setup.
You might see something like this: "Warning: /usr/local/lib/pkgconfig isn't writable."
It will give you the advice that: "You should probably chown /usr/local/lib/pkgconfig".
This means: sudo chown {your-user-name} /usr/local/lib/pkgconfig
Then you will need to link the files with this: brew link libyaml
If this does not work hopefully the output of brew doctor
will give you enough to continue the search.
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