AssemblyLinearVelocity not working on players. Any explanations ?

It's because of Network Ownership. All limbs of the dummies/NPCs are owned by the server, while the limbs of the players are owned by the players themselves. Because of that, you need to use AssemblyLinearVelocity in the Client-side.

You can use RemoteEvents to communicate with the client, or temporarily set the NetworkOwnership to the server.

• Using a RemoteEvent

-- Server (Normal Script in Workspace or ServerScriptService) --
local Players = game:GetService("Players")
local ReplicatedStorage = game:GetService("ReplicatedStorage")
local RemoteEvent = ReplicatedStorage.RemoteEvent -- imagine that there's a remote event named "RemoteEvent" in ReplicatedStorage
-- lines of code later...
local player = Players:GetPlayerFromCharacter(hithrp.Parent)
if player then -- if it's a real player
    RemoteEvent:FireClient(player) -- you have to define the "RemoteEvent" variable and the "player" variable yourself so this will work
end
-- the rest of the code

-- Client (Local Script in StarterPlayerScripts) --
local Players = game:GetService("Players")
local Player = Players.LocalPlayer

local ReplicatedStorage = game:GetService("ReplicatedStorage")
local RemoteEvent = ReplicatedStorage:WaitForChild("RemoteEvent") -- imagine that there's a remote event named "RemoteEvent" in ReplicatedStorage

RemoteEvent.OnClientEvent:Connect(function()
    local character = player.Character or player.CharacterAdded:Wait()
    local hrp = character:WaitForChild("HumanoidRootPart")
    hrp.AssemblyLinearVelocity = Vector3.new(100, 60, 0)
end)

• Setting NetworkOwnership to Server temporarily

-- Server (Normal Script in Workspace or ServerScriptService) --
local Players = game:GetService("Players")
-- lines of code later...
local player = Players:GetPlayerFromCharacter(hithrp.Parent)
if player then -- if it's a real player
    hithrp:SetNetworkOwnership(nil) -- sets NetworkOwnership to server
    hithrp.AssemblyLinearVelocity = Vector3.new(100,60,0)
    task.wait(2)
    hithrp:SetNetworkOwnership(player) -- sets NetworkOwnership back to original owner (aka the client or the player themself)
end
-- the rest of the code

ALSO DO NOT COPY AND PASTE THIS ANSWER DIRECTLY TO YOUR CODE BECAUSE IT WON'T WORK. THIS JUST SHOWS OF WHAT YOU WILL DO AND EDIT IN YOUR CODE TO MAKE IT WORK.