Use a T-6 or T-7 Torx bit to adjust the tightness of the little screw highlighted in red in the bottom-left corner of the device (top-down).

If the right click works but the left click doesn't, you'll need to turn the screw clockwise.

Note: right and left clicks are determined by the Trackpad top, not the view from the bottom as you are adjust the screw.

If neither the right click nor left click work, turn it counter-clockwise if there is no movement at all, and clockwise if there is movement but it is too soft to click.