I really wanted to use this addon so I fixed it.
Open file Open-Sesame_Vanilla.lua in notepad.
Line 3 should like like:
OpenSesame:SetScript('OnEvent', function(self, event, ...) selfevent end)

After that line add the following lines:
local GetContainerNumSlots = C_Container.GetContainerNumSlots
local GetContainerItemID = C_Container.GetContainerItemID
local UseContainerItem = C_Container.UseContainerItem

This did not fix it for me unfortunately

What other errors are you getting? I just installed using the Curseforge client and then added those lines. If I get lua errors for other addons, Open-Sesame will sometimes stop working and I have to reloadui. It is opening clams so far for me. I'm below level 30.
Here is what the top of my file looks like:

local OpenSesame = CreateFrame('Frame')

OpenSesame:SetScript('OnEvent', function(self, event, ...) selfevent end)

local GetContainerNumSlots = C_Container.GetContainerNumSlots
local GetContainerItemID = C_Container.GetContainerItemID
local UseContainerItem = C_Container.UseContainerItem

-- If you want to see Startup Messages, simply un-comment the next line.
-- print("|cff00FF80OpenSesame : Loaded WoW Classic Era")