That's an Eagle oddity called "Implicit Power Pins". Someone thought that it was onerous to wire Vcc and Vss pins to every gate, so they came up with this solution.
As the error message suggests, there's an implicit power pin for Vss. Select the part, and then issue the
invoke command. You'll be presented with a simple pin symbol which you can drop anywhere on your schematic. Even thought the schematic symbols are distinct, they're mapped to the same physical part.
Alternatively, the Eagle way is to wire it up by creating a net called "Vss" somewhere in your schematic, perhaps on a decoupling cap. This is automatically connected to all the implicit Vss pins on any ICs which use this mechanism. Use the appropriate power pins in the 'supply' library to make this connection.
Personally, I edit symbols which are built like this. Open a library you want your fixed part to go into. Then, in the control window, open the library containing this part, and right-click -> 'copy to library'. Edit the symbol to include power and ground pins, and edit the part to use these new pins over the implicit ones.