The error means that the function requires a float (number), but you gave it a Vector3 (script.Parent.Part.Size). Your second argument should just be a number, so get the size on one of the axis because I’m assuming the part you’re filling is a sphere. script.Parent.Part.Size.X.
I also think the first argument should be a Vector3 Position, not a CFrame. You can look at what arguments the FillBall function requires on the wiki.