I'm no expert. Steam's How To pages include the following:
https://support.steampowered.com/kb_article.php?ref=3134-TIAL-4638
https://support.steampowered.com/kb_article.php?ref=3134-TIAL-4638
How to remove and refresh your Steam files
Mac OSX
Completely exit from the Steam client.
Go to the folder Users/[username]/Library/Application Support/Steam
Delete all of the files in this folder except for SteamApps.
Restart your computer.
Launch Steam and re-test the issue.