Okay, I don’t know everything about splines, but I can tell you a few things about them. 1) In your blueprint, click the “add spline mesh component” node. In the detail panel to your left (in the bp), make sure that it is “Moveable” and not static. 2) In the for loop in the picture above that you posted, it is important to note that because spline point indexes start at 0, when you add 1 to the point index you are indeed returning an invalid point - because - it doesn’t exist yet. So to solve this, after you’ve gotten the number of spline points, subtract one (- 1) from it - this is the number you plug into the “last index” in your for-loop. Lastly, as I see also in the picture you’re using points for location and tangents rather than a distance scale and segments… try viewing this tutorial. It may answer some questions for you, but as I’ve found the deeper you dig the more questions you have 
Hope it helps