Not awarding XP when I touch the part?

By writing for player.Parent you are referring to Players folder, and that's a mistake. You should also refer to the Player, and not the character.

Here is something I would prefer you use.

script.Parent.Touched:connect(function(hit) -- This makes the function happen when the brick is touched
    if game.Players:GetPlayerFromCharacter(hit.Parent) then -- The function GetPlayerFromCharacter refers to the Player, we use an if statement to makes sure it's a real player, 
        local player = game.Players:GetPlayerFromCharacter(hit.Parent) -- A variable referring to the Player
        if player:FindFirstChild("leaderstats") then -- Check if leaderstats exist
            player.leaderstats.exp.Value = player.leaderstats.exp.Value + 15 --  increases the value
        end
        script.Parent:Destroy()  -- This is to remove the part that is touched, in case you don't want the player to touched again
    end
end)

The Character is the part of the player that is under the Player folder The Player is the part of the player that is under Workspace