this is something relatively obnoxious that happens-

you worked on the model in blender, and then overwrote it in unity.

then go to build, and it gives you this error.

so you go to the rig window, but everything is assigned as it should be. And now that you have confirmed that, it now goes away once you try to upload again.

It's just a kind of annoying bug that blocks you and forces you to open and close the rig config window before you can proceed. Mind you, it doesn't always do this.