Saved variables not working

[Codger]

I've created a very simple addon to see how saved variables work but the saved variables are not working.
Here is what I have so far:

My .toc file:

## Title: MyAddon
## Interface: 100206
## Version: 1.0
## Author: 
## Notes: A simple WoW addon using a database
## SavedVariablesPerCharacter: MyAddonDb
MyAddon.lua

and my lua code is:

-- Create a saved variable to store last time
local lastLogoffTime = nil
 
-- Create a new frame
local frame = CreateFrame("Frame")
 
-- Load last logoff time from saved variable
local function LoadLastLogoffTime()
    if MyAddonDB then
        lastLogoffTime = MyAddonDB.lastLogoffTime
        print("Last logoff time loaded:", lastLogoffTime)
    else
        print("Trying to load logoff time but MyAddonDB is nil.")
    end
end
 
-- Save last time to saved variable
local function SaveLastLogoffTime(time)
    if not MyAddonDB then 
        print("Trying to save logoff time but MyAddonDB is nil") 
    else
        MyAddonDB.lastLogoffTime = time
    end
end
 
-- Hook the Logout event to update last logoff time
frame:RegisterEvent("PLAYER_LOGIN")
frame:RegisterEvent("PLAYER_LOGOUT")
frame:SetScript("OnEvent", function(self, event)
    if event == "PLAYER_LOGIN" then
        if not MyAddonDB then
            MyAddonDB = {}
            print ("MyAddonDB was nil")
        end
        LoadLastLogoffTime()
    elseif event == "PLAYER_LOGOUT" then
        SaveLastLogoffTime()
    end
end)
 
-- Slash command handler function
local function SlashCommandHandler(msg)
    LoadLastLogoffTime()
end
 
-- Register slash command
SLASH_MYADDON1 = "/myaddon"
SlashCmdList["MYADDON"] = SlashCommandHandler

When I login I see these messages:
My AddonDB was nil
Lastlogofftimeloaded: nil

and after logging out, the contents of the saved variable file 'MyAddon.lua' is:
MyAddonDb = nil

I've looked at so many posts but can't seem to figure out what I'm doing wrong.
I've logged out of my character and deleted the WTF saved variables file - didn't help.
I've restarted my computer in case the file was somehow preventing writing but no help.
I appreciate any help you can provide.