Does anyone know how to help me with this issue im having with my NPC follow player script?

Here is a better way to get the closest player:

local closestM = math.huge --To make sure every number is less than this
local closestP = nil

for i,v in pairs(game.Players:GetPlayers()) do
    local mag = (NPC.HumanoidRootPart.Position - v.Character.HumanoidRootPart.Position).magnitude
    if mag < closestM then
        closestM = mag
        closestP = v
    end
end
--you can use the closestP as you want

I replaced the getClosestPlayer function with mine and every PrimaryPart with HumanoidRootPart. So now your script should look like this:

local NPC = game.Workspace.Matt
local pathfinding_service = game:GetService("PathfindingService")
function getClosestPlayer()
    local closest_distance = math.huge
    local closest_player = nil

    for i,v in pairs(game.Players:GetPlayers()) do
        local mag = (NPC.HumanoidRootPart.Position - v.Character.HumanoidRootPart.Position).magnitude
        if mag < closest_distance then
            closest_distance = mag
            closest_player = v
        end
    end
    return  closest_player, closest_distance
end

while true do
    local path = pathfinding_service:CreatePath()
    local player, distance = getClosestPlayer()
    local player_char = player.Character
    if player and distance > 10 then
        path:ComputeAsync(NPC.HumanoidRootPart.Position, player_char.HumanoidRootPart.Position)
        local waypoints = path:GetWaypoints()
        for _, waypoint in pairs(waypoints) do
            NPC.Humanoid.MoveTo(waypoint.Position)
            while true do
                local distance = (NPC.HumanoidRootPart.Position - waypoint.Position).Magnitude
                local distance_from_player = (NPC.HumanoidRootPart.Position - player_char.HumanoidRootPart.Position).Magnitude
                if distance < 5 then
                    break
                end
                if distance_from_player < 10 then
                    NPC.Humanoid:MoveTo((NPC.HumanoidRootPart.CFrame*CFrame.new(0,0,-3)).p)
                    break
                end
                wait()              
            end
        end
    end
    wait()
end

Problem

The reason it didn't work was because there was no HumanoidRootPart, hence the error.

Check by clicking on this hyperlink

Make sure your NPC has a HumanoidRootPart by checking.

Fixed Script:

local NPC = game.Workspace.Matt
local pathfinding_service = game:GetService("PathfindingService")

function getClosestPlayer()
    local closest_player, closest_distance = nil, 200
    for i, player in pairs(workspace:GetChildren()) do
        if player:FindFirstChild("Humanoid") and player ~= NPC then
            local distance = (NPC.PrimaryPart.Position - player.PrimaryPart.Position).Magnitude
            if distance < closest_distance then
                closest_player = player
                closest_distance = distance
            end
        end
    end
    return  closest_player, closest_distance
end

while true do
    local path = pathfinding_service:CreatePath()
    local player, distance = getClosestPlayer()
    if player and distance > 10 then
        path:ComputeAsync(NPC.HumanoidRootPart.Position, player.PrimaryPart.Position)
        local waypoints = path:GetWaypoints()
        for _, waypoint in pairs(waypoints) do
            NPC.Humanoid:MoveTo(waypoint.Position)
            while true do
                local distance = (NPC.PrimaryPart.Position - waypoint.Position).Magnitude
                local distance_from_player = (NPC.PrimaryPart.Position - player.PrimaryPart.Position).Magnitude
                if distance < 5 then
                    break
                end
                if distance_from_player < 10 then
                    NPC.Humanoid:MoveTo((NPC.HumanoidRootPart.CFrame*CFrame.new(0,0,-3)).p)
                    break
                end
                wait()              
            end
        end
    end
    wait()
end

Recommendation(s):

• Use :MoveTo because .MoveTo isn't a function.

Also, ensure that your script is in Workspace.

Thanks, JesseSong