Help on this script with IntValues?

Try this.

game.Players.PlayerAdded:connect(function(player)
    player.CharacterAdded:wait() --Waits until the CharacterAdded event has fired.
    wait(1)
    local Mana=Instance.new("IntValue") -- New IntValue
    Mana.Parent = player
    Mana.Name = "Mana" --Give a name to the IntValue, so you can easily identify it in other scripts.
    local MAXMana=Instance.new("IntValue")
    MAXMana.Parent = player
    MAXMana.Name = "MAXMana" --Give a name to the IntValue, so you can easily identify it in other scripts.
    local level= player:WaItForChild("leaderstats").Lvl.Value --Wait for leaderstats to become available. 
    MAXMana.Value = level*20 
    Mana.Value = level*20 -- I have other scripts that would take away Mana upon casting a spell
    while wait() do
        if Mana.Value < MAXMana.Value then -- If your MANA is below your MAX then replenish
            wait(0.5)
            Mana.Value = Mana.Value + 1
        elseif Mana.Value > MAXMana.Value then --If your MANA is above your MAX somehow then set it to the max.
            Mana.Value = MAXMana.Value
        else --If any of the above conditions aren't true then.
            repeat wait() until (Mana.Value > MAXMana.Value) or (not Mana.Value > MAXMana.Value) --Wait until MANA is over or below MAX.
        end 
    end
end)

Make sure the script is in Workspace or ServerScriptStorage.

Mana.Parent and MAXMana.Parent should be set to player and at local level you can use player.leaderstats.Lvl.Value. Since you're running it inside a PlayerAdded event, you can easily find anything related to the player.

I suggest you make a model inside player holding these things.

local Stats  = Instance.new("Model", player)  -- Sets the parent right away
local Mana = Instance.new("IntValue", Stats)
local MAXMana = Instance.new("IntValue", Stats)

Stats.Name = "Stats"
Mana.Name = "Mana"
MAXMana.Name = "MAXMana"

Then with the while loops, you can do something like this

while wait(0.5) do
    if Mana.Value<MAXMana.Value then
        Mana.Value = Mana.Value+1
    else
        Mana.Value = MAXMana.Value
    end
end

Now you have them both in the same loop which repeats itself every 0.5 second. At your script the second loop won't run because it's stuck at the first loop.

Don't keep player values in the workspace because if there is more than one player, then there will be more than one "Mana" and "MAXMana", which will cause scripts that change those values to not work properly. The more efficient way of doing it is by parenting the Mana and MAXMana values to the player when the player enters the game.

game.Players.PlayerAdded:connect(function(Plyr)
    Plyr.CharacterAdded:wait() --waits until the CharacterAdded event fires
    local Mana = Instance.New("IntValue") -- New IntValue
    Mana.Name = "Mana" --Change their name so that other scripts can find the values
    Mana.Parent = Plyr
    local MAXMana = Instance.new("IntValue")
    MAXMana.Name = "MAXMana"
    MAXMana.Parent = Plyr
    local level = Plyr:WaitForChild("leaderstats").Lvl.Value --Use WaitForChild to prevent errors
    MAXMana.Value = level * 20 --Set the value to this number, not the MAXMana itself
    Mana = level * 20 -- I have other scripts that would take away Mana upon casting a spell
    while Plyr do --the loop will run as long as the player is in the game
        wait(0.5)
        Mana.Value = (Mana.Value >= MAXMana.Value and MAXMana.Value or Mana.Value + 1) --I used a ternary operation to get rid of the second while true do loop 
    end
end)

NOTE: Make sure the script is in the Workspace. If it isn't, it won't work