The actual reason why is it says that character isn’t a valid member of meshpart is because you’re actually saying:
MeshPart.Character.Humanoid.WalkSpeed = plr.Character.Humanoid.WalkSpeed - .4
For other problems you have i’ll discuss it in my head later so i can have some plausible solutions.
Edit: The other errors earlier that also said that character is not a valid member of part is also the issue with getting the part, not the parent.
Part.Character.Humanoid.WalkSpeed = plr.Character.Humanoid.WalkSpeed - .4