There are 4 holes for screws to hold the T8 lead screw nut. I used 4 screws. I snugged them up.
When I tried to move the Z-axis by hand (by turning the coupler with my fingers), it was fairly easy until I got to within 1/2" of the top, in which case it became very difficult.
After a bunch of fiddling with the gantry (taking it apart, swapping parts), I had an ‘ah ha’ moment, and backed off the 4 screws slightly. That fixed the problem.
The documentation mentions that you don’t need all 4 screws, that the screws are just there to stop the nut from spinning. In order to save someone else a bunch of time the documentation could explicitly suggest not snugging up the screws because it could result in the nut being not perfectly in line with the lead screw and cause it to bind when the gantry gets close to the top.