[Dridzt]

local button = CreateFrame("Button", nil, UIParent, "UIPanelButtonTemplate")
button:SetSize(100,20)
button:SetPoint("CENTER")
button:RegisterForClicks("AnyUp")
button:SetText("Start")
button:SetScript("OnClick", function(self,btn)
  if self:GetText()=="Start" then
    self:SetText("Stop")
  else
    self:SetText("Start")
  end
end)
button:SetMovable(true)
button:RegisterForDrag("LeftButton")
button:SetScript("OnDragStart", button.StartMoving)
button:SetScript("OnDragStop", button.StopMovingOrSizing)
button:SetScript("OnHide", button.StopMovingOrSizing)
button:Show()

This should make a minimal button for testing but your error mentions line 641, meaning there's a ton of code you're not showing.

The most probable cause is that fs does not exist inside StopStart because it is defined as a local in another scope.