Hi.
It is because rigidbody uses physics and overrides the itween move
You can use for example 'Set Velocity' (every frame)
And you might want to make a parent (cube or capsule) and place the rigidbody there.
You can disable the mesh to make it invisible.
Then also do some constrains on the rotation so it wont roll over.
Then drop in your 'ground enemy'
and do the movement on the parent.
Parenting is a very common thing with character (and other things) as it give a lot more freedom of what to do with them.
What course you where following?