On the left, where it says Translucent you need to change to Masked, Translucent is for when you need multiple levels of opacity which is what the Opacity slot is for. Masked is black and white either fully visible or not, it has the advantage of having better performance and fixes some issues with layering.
Once you’ve switched it to Masked then you would plug your alpha map into the Opacity Mask slot
If your diffuse map has an alpha channel already in it, then you can use it from the last output on your texture node the one after the Blue channel output.