Trying to learn about the Pathfinding Service but my MoveTo is not working?

It's because you're connecting the events after the while loop. As the while loop doesn't let anything else after it run, the events will never connect.

Move the connects to before the while loop, and it should be fine.

path.Blocked:Connect(onPathBlocked)

NPC_Humanoid.MoveToFinished:Connect(onWaypointReached)

while true do
    wait(5);
    local nearestPlayer = findNearestPlayer(NPC.HumanoidRootPart.Position);

    if nearestPlayer then
        followPath(nearestPlayer);
    end
end

Hello,

local pathFindingService = game:GetService("PathfindingService");
local path = pathFindingService:CreatePath();
local NPC = script.Parent;
local NPC_Humanoid = NPC.Humanoid;

local waypoints;
local currentWaypointIndex = 1;

local function findNearestPlayer(position)
    local lowest = math.huge;
    local nearestPlayer;

    for _, v in pairs(game.Players:GetPlayers()) do
        if v then
            if v.Character then
                local distance = v:DistanceFromCharacter(position);
                if distance <= lowest then
                    lowest = distance;
                    nearestPlayer = v;
                    print(nearestPlayer.Name.." is closest player at a distance of :".. distance );
                    return nearestPlayer;
                end
            end
        end
    end
end

local function followPath(player)
    path:ComputeAsync(NPC.HumanoidRootPart.Position,    player.Character.HumanoidRootPart.Position);

    waypoints = {};
    if path.Status == Enum.PathStatus.Success then
        waypoints = path:GetWaypoints();
        -- currentWaypointIndex = 1; -- You are declaring the index being 1.
        print(currentWaypointIndex)
        NPC_Humanoid:MoveTo(waypoints[currentWaypointIndex].Position);
    else
        NPC_Humanoid:MoveTo(NPC.HumanoidRootPart.Position);
    end

end

local function onWaypointReached(reached)
    if reached and currentWaypointIndex < #waypoints then
        currentWaypointIndex = currentWaypointIndex + 1;
    NPC_Humanoid:MoveTo(waypoints[currentWaypointIndex].Position);
    end
end

local function onPathBlocked(blockedWaypoint)

    if blockedWaypoint > currentWaypointIndex then
        local nearestPlayer = findNearestPlayer(NPC.HumanoidRootPart.Position);
        followPath(nearestPlayer);
    end
end


while true do
    wait(15);
    local nearestPlayer = findNearestPlayer(NPC.HumanoidRootPart.Position);
    if nearestPlayer then
        followPath(nearestPlayer);
    end
end


path.Blocked:Connect(onPathBlocked)

NPC_Humanoid.MoveToFinished:Connect(onWaypointReached)

So, having redacted the mistake, I'll proceed to explain.

In the while loop, you are checking for the nearestPlayer and "following" it with the function followPath, for now everything was good, but on Line 24, you were declaring the currentWaypointIndex to be 1, constantly as it is inside of a while loop.

To counter this, we make currentWaypointIndex a "global" in the scope of this script; Line 7.

This should fix your issue, any further questions DM me.

PS:Please accept this as an answer if this helped you.