It seems that there is no material applied.
How did you place the vegetation pack in UE4 ? Did you just copy, or migrate, or added it from the launcher ? (marketplace asset or some stuff from TurboSquid,etc)
Either the vegaetation pack came without materials (which I can hardly imagine), or you some connections are broken.
Check your vegetatin packs for the materialscontent and, if neccessary, reassign them to the meshes.
If a material also shows the gray default material, then a shader might be broken. Open the material to see whats going on there.