NullReferenceException: Object reference not set to an instance of an object at CharacterMotorSimple.ApplyFallDamage (Single clippedCrouchVelocity) [0x00000] in :0 at CharacterMotorSimple.Simulate (Single time) [0x00000] in :0 at NetMovementServerPlayerOwned.Tick (Single tickSize) [0x00000] in :0 at NetworkEntityManagerPlayerServer.Tick () [0x00000] in :0 at NetworkEntityManagerPlayerServer.Update () [0x00000] in :0

@Wulf

I've installed a hurtworld server, 
Which is on a host, 
I've already created a support ticket I asked them  why this is happing after a few minutes they reply back, 
And they told me that it's the mod itself, 

Doesn't appear to be coming from Oxide, likely a plugin; we don't modify the game directly for things like fall damage. The first step for you would be to remove all plugins to see if the error persists.

Also, in most cases, hosts will not be able to assist with game-related errors.

Can it also be possible that a workshop mod isn't updated and that is why I'm not be able to move around?
Because I haven't installed any plugins yet 

Yes, very possible.

Make sure you ain't using any mods/plugins which change player movement.