Once the splines mate and the coupler slides even a short distance on the shaft you can let go of the spring loaded bolt and the retention bolt should snap into place when it gets to the retention groove on the shaft.
If you can't get the coupler to mate with the shaft splines or slide less than an inch, that normally means that the retention bolt is not pressed far enough in to allow the shaft to seat.
If the distance between the retention bolt and end of the coupler is too far, or if there is something on the shaft or in the spline grooves, that could be restricting the ability of the retention bolt to line up with the retention groove.
Dumb question, but I assume that the chain is long enough to allow the shaft to fully extend.
Beverly Howard