On the Mac, if you're using Postgres.app, the pg_config file is in your /Applications/Postgres.app/Contents/Versions/<current_version>/bin
directory. That'll need to be added to your system path to fix this error, like this:
export PATH=$PATH:/Applications/Postgres.app/Contents/Versions/<current_version>/bin
So for example, if the current Postgres.app version is 9.5, this export line would be:
export PATH=$PATH:/Applications/Postgres.app/Contents/Versions/9.5/bin
With more recent versions of the Postgres.app (> 9.5?), you can simply add "latest" in place of the version number, like so:
export PATH=$PATH:/Applications/Postgres.app/Contents/Versions/latest/bin
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