The route plotter will only plot jump distances based on a full tank.
So route plotting can fail if, somewhere along that course, it can't find a way across a gap that is under 29.95Ly
As I understand it, for each step in the course, it will only consider next hops within 180 degrees of the destination, so it won't backtrack or jump sideways to get across a gap.
You can often manually get across gaps by selecting a star as your target but don't plot a course there. If the distance is under your current jump range with your current fuel level, you can just jump without plotting a course.
That can get you across gaps that are over your full-tank jump range but under your max jump range.
As Ken pointed out, 29Ly should be OK at + or - 500Ly but another thing that can cause route plotting to fail is crossing permit locked regions.
If there are a few permit locked system in your way the plotter has to skip them and plot a jump past them and you may not have the range for that.
You can tell if a system is locked by selecting it in the Galaxy map. If it's locked you won't be able to select it.
Try plotting shorter courses. Instead of all the way there or 1000Ly, try a 200Ly course in that direction. If you can't plot that, try 100Ly, or 50Ly.
Eventually you'll get close to whatever is blocking your way and you can manually plan a course around it.