Unable to install Chrome or Safari extensions after creating new user account
You don't describe how the Safari extensions fail to install, but if you had the same problem as I you would first get a dialog box saying that something went wrong, and the extension install button is then replaced with a red "installation failed" message. 

I solved this by going into the keychain manager preferences and resetting the default keychain. After that all extensions installed normally.

For some reason, every time I started Safari it asked for the password to the default keychain, and being a freshly installed OS I just typed the only password I had -- my user password -- which it didn't accept. Apparently Safari just couldn't verify the extensions' signatures.

I got the idea while reading Repeated &quot;Safari wants to use the &#39;Local Items&#39; keychain&quot; alert