Material blends in 4.18

Hi all.

I’m very new to Unreal, and maybe I should stick to 4.17 for the time being but AR support grabbed me. I am working on some landscapes and want to paint different materials onto meshes. I’ve followed this page but when it comes to painting the vertices, the option for Blend Weights mode isn’t in the modes window. Is this a 4.18 change? I’m happy to share my project file.