VectorVelocity.X/Y/Z takes numbers, not vector3s. If you want to do it like that, then do...
bullet.LinearVelocity.VectorVelocity = mouse.Hit
or
bullet.LinearVelocity.VectorVelocity.X = mouse.Hit.X
bullet.LinearVelocity.VectorVelocity.Y = mouse.Hit.Y
bullet.LinearVelocity.VectorVelocity.Z = mouse.Hit.Z
I'd also double-check that mouse.Hit is actually a vector3 and not a raycastResult or something like that
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