I am running a simple analysis of a rod fixed on one end, and I am trying to find the minimum force required to the material to yield. I am getting a large displacements error message. If I continue with the analysis will my results for von Mises yield be accurate? What exactly does this error message mean? Thanks!

If you can imagine, as your rod deflects the geometry changes. As it changes a lot, the results become unreliable.

You need to lower your load a bit. If you're just looking for the yield point, find the load that gets the stress to, say, 50% yield. Then you know the yield load is 2X your applied load.