Linear velocity requires an attachment. To use it, you first have to insert an attachment in the part you want to move and then set the LinearVelocity.Attachment0 to that attachment.
A simple example can be:
-- you need to define the player first
local HRP = player.Character.HumanoidRootPart
local Part = Instance.new("Part", workspace) -- the part to move
Part.CFrame = HRP.CFrame * CFrame.new(0,0,-3) -- Putting the part infront of the player
local Attachment = Instance.new("Attachment", Part) -- the attachment used in Linearvelocity
local LV = Instance.new("LinearVelocity", Attachment) -- creating the linear velocity
LV.MaxForce = math.huge -- no need to worry about this
LV.VectorVelocity = HRP.CFrame.lookVector * 100 -- change 100 with how fast you want the projectile to go
LV.Attachment0 = Attachment -- setting the attachment
game.Debris:AddItem(Part, 2) -- deletes the moving part after 2 second