Bit of an odd one this. I'm using a laptop to play terraria. Hard clicking works as expected, but I can't tap click (which I can do with every other game I have).
Yeesh, you play with a touchpad? I'd recommend plugging in a mouse if you can -- I would imagine it's pretty easy to mess up trying to do certain things using a touch-based interface. My only computer is a laptop, and I wouldn't dream of playing anything with the trackpad that wasn't turn-based -- and even then, only in a pinch.
But to more directly answer your question, I'd say maybe the game auto disables tap-to-click? Or something? You could always try Alt + Tabbing away from Terraria
and checking your touchpad settings while the game's running, just to make sure it's not doing something. (On my Toshiba, there's an icon for the trackpad software in the system tray, and if I right-click this, one of the options appearing in the context menu that pops up is "Tap to click". Obviously, this might be different on your system, but it's something to check.)