If space- time is a manifold with no physical reality why is it Said were moving in spacetime in Universe ?

Answer 1

Spacetime is NOT a manifold with no physical reality.

A manifold is a manthematical construct that can be used to describe spacetime. Just like words can be used. You would not say "because spacetime is just a word, blah blah".

A manifold is used because in general relatiivty it is not possible to construct Euclidean spacetimes that extend to infinity.