How to connect the tristate pin to GND


This is why ERC warns you about the potential conflict, but does permit you to make the PCB


I think you should first look at what is the need for unused inputs to be connected to one state or the other.

The reason is that un-connected inputs can bounce around between Hi and low putting the internal circuitry in a linear condition. Not good for logic devices.

I see you your issue is. Your unused pins could be either inputs (which should be grounded) or outputs which should be open.

I recommend a resistor to ground, perhaps 10k. This will keep the inputs from floating, yet not unduly load the pin when it is an output.