Server Script not changing values inside local player?

Hello. So, I have this server script which is supossed to change some values inside the local player when it receives a remoteEvent, but for some reason it does not change anything. Can anyone help me out?

Here's the script: (oh, sorry if it's a little bit messy, I am new to making my own projects without tutorials.)

game.ReplicatedStorage.TaskFinished.OnServerEvent:Connect(function(player)
	local ongoingtasks = player.values.ongoingtasks.Value
	local finishedtasks = player.values.finishedtasks.Value
	ongoingtasks = false -- THis does not work.
	finishedtasks = finishedtasks + 1 --This does not work.
	player.PlayerGui.Tasks.objective.Text = "Objective: None"
	player.PlayerGui.Tasks.main.current_task.Text = "Current task: None"
	print("Finished task correctly.")
end)

local ongoingtasks = player.values.ongoingtasks.Value

you are storing the value inside the ongoingtasks BoolValue object's .Value property inside the variable ongoingtasks. Changing the value inside ongoingtasks does not change the value inside the actual BoolValue inside the player. To do so, you have to set its .Value property like so:

player.values.ongoingtasks.Value = false

To make the code more readable, I would store the player.values.ongoingtasks BoolValue object inside a variable:

local ongoingtasks = player.values.ongoingtasks
ongoingtasks.Value = false

The same concept can be applied to finishedtasks.

local finishedtasks = player.values.finishedtasks
finishedtasks.Value = finishedtasks.Value + 1

Here's what your code should look like:

game.ReplicatedStorage.TaskFinished.OnServerEvent:Connect(function(player)
    local ongoingtasks = player.values.ongoingtasks
    local finishedtasks = player.values.finishedtasks
    ongoingtasks.Value = false -- THis does not work.
    finishedtasks.Value = finishedtasks.Value + 1 --This does not work.
    player.PlayerGui.Tasks.objective.Text = "Objective: None"
    player.PlayerGui.Tasks.main.current_task.Text = "Current task: None"
    print("Finished task correctly.")
end)

Unrelated to your question but I see that you're modifying the player's gui through a server script. I recommend you use a local script when it comes to modifying anything related to the player.PlayerGui as a general rule of thumb, since guis by nature are local.

For the record, you can't change those values inside a local script. It will only show up for the player, and other scripts won't see it.