Welcome to UE! From what I could gather around the community, that warning is due the way the material blending function works, its limited to 3 layers (most times, a base and two blends). Anything that surpasses that limit will result in the warning, and the blending will not work as intended.
To get rid of the warning, keep 3 material layers only, and delete any extra slots. Another thread with the same scenario reached the same conclusion, as seen below: