Do you have permanent Struts mounted, or, are you in Bootstrap mode using the small temporary printed Strut parts and possibly have a wonky/racked setup that adds to Z friction/resistance? Go slow until you have permanent Struts installed , they help provide correct alignment (reducing binding risk) as well as providing rigidity.
Did you lube the leadscrews really well? Make sure temp stickers are adhered first.
With the YZ plates as vertical as possible, and Beam as low as possible…
Try loosening bolt between Z Nut and Z Stub. Play with the leadscrew angle, find out how far the leadscrew can slide withing the Stub’s slotted hole, then position in the center (or wherever the least bend force on coupler is)… Thank/blame Jamie for this technique.
Then, loosely snug the bolt connecting Nut and Stub. Snug just enough to engage the Nylock, don’t tighten to the point that pressure gets applied between the parts that would add friction/resistance to Z travel.