Datastore not saving or retrieving? Please help!

I think the problem might be the fact when the data is being retrieved for the very first time, GetAsync returns nil.. your code doesn't check to make sure its not nil.. when you set days.Value to a nil value, it will halt the code indefinitely.. so do something like this:

local CollectionService = game:GetSerice("CollectionService")
local DataService = game:GetService("DataStoreService")
local DataStore = DataService:GetDataStore("Doesnnotd Matterrigh now")

game.Players.PlayerAdded:Connect(function(player)

    local days = player:WaitForChild("leaderstats").Days

    local Days_Data


    local success, errorMessage = pcall(function()
        Days_Data = DataStore:GetAsync(player.UserId.. "days") --(key, keyword)  --you should make it a keyword, rather than a value
    end)

    if success then
        days.Value = Days_Data  or 0
    CollectionService:AddTag(days, "data_loaded")
        print("Retrived")
    elseif errorMessage then
        warn(errorMessage)
    end
end)

game.Players.PlayerRemoving:Connect(function(player)
    local days = player.leaderstats.Days

    if(not CollectionService:HasTag(days, "data_loaded") then
    --this ensures data is saved only is it has been loaded from datastore, to avoid data loss
    return
    end

    local success, errorMessage = pcall(function()
        DataStore:SetAsync(player.UserId.. "days", days.Value) --(key, keyword, value)
    end)

    if success then
        print(player," Has Left")
        print("All Saved")
        print(days.Value)
    else
        warn(errorMessage) --warn is like print, except in orange text, showing where the warning is
    end
end)

Maybe you have API Service disabled? Also, DataStore doesn't work inside Roblox Studio.

If you go to "Game Settings" in the "Home Tab", then go to "Options", and "Enable Studio API services", and set that to true. Your place must also be published.

Go to game settings Then turn on Enable Studio API

try this

-- Set up table to return to any script that requires this module script
local PlayerStatManager = {}

local DataStoreService = game:GetService("DataStoreService")
local playerData = DataStoreService:GetDataStore("PlayerData")

-- Table to hold player information for the current session
local sessionData = {}

local AUTOSAVE_INTERVAL = 60

-- Function that other scripts can call to change a player's stats
function PlayerStatManager:ChangeStat(player, statName, value)
    local playerUserId = "Player_" .. player.UserId
    assert(typeof(sessionData[playerUserId][statName]) == typeof(value), "ChangeStat error: types do not match")
    if typeof(sessionData[playerUserId][statName]) == "number" then
        sessionData[playerUserId][statName] = sessionData[playerUserId][statName] + value
    else
        sessionData[playerUserId][statName] = value
    end
end

-- Function to add player to the "sessionData" table
local function setupPlayerData(player)
    local playerUserId = "Player_" .. player.UserId
    local success, data = pcall(function()
        return playerData:GetAsync(playerUserId)
    end)
    if success then
        if data then
            -- Data exists for this player
            sessionData[playerUserId] = data
        else
            -- Data store is working, but no current data for this player
            sessionData[playerUserId] = {Money=0, Experience=0}
        end
    else
        warn("Cannot access data store for player!")
    end
end

-- Function to save player's data
local function savePlayerData(playerUserId)
    if sessionData[playerUserId] then
        local success, err = pcall(function()
            playerData:SetAsync(playerUserId, sessionData[playerUserId])
        end)
        if not success then
            warn("Cannot save data for player!")
        end
    end
end

-- Function to save player data on exit
local function saveOnExit(player)
    local playerUserId = "Player_" .. player.UserId
    savePlayerData(playerUserId)
end

-- Function to periodically save player data
local function autoSave()
    while wait(AUTOSAVE_INTERVAL) do
        for playerUserId, data in pairs(sessionData) do
            savePlayerData(playerUserId)
        end
    end
end

-- Start running "autoSave()" function in the background
spawn(autoSave)

-- Connect "setupPlayerData()" function to "PlayerAdded" event
game.Players.PlayerAdded:Connect(setupPlayerData)

-- Connect "saveOnExit()" function to "PlayerRemoving" event
game.Players.PlayerRemoving:Connect(saveOnExit)

return PlayerStatManager

This problem also happened to me, the way I fixed it was by turning off the DataStore created by my admin commands I was using, and also double check that Studio has access to API Services by going to Game Settings, Options, and then turning the option on.