Make the character move with the object with Spline Animate

I made this cart by slapping five cubes together but now I don’t know why the character keeps falling off.

Is there anyway I can keep the player armature anchored inside the cart?

Currently when I click W button the cart begins to move, it is currently set up using the old input system a.k.a. Input-Manager.

This is the game scene

Reddit link with Video:

Any help will be appreciated… Thanks in advance