[Fizzlemizz]

They work but the Yes will error on click because StartGame is declared as a local function and it appears in your code below the StaticPopupDialogs["DEATHDICE_CONFIRM"] declarations meaning it's not in scope (can't be seen).

Either move the function above the popup declaration (probably best) or remove the local declaration from StartGame making it global (in which case, StartGame is probably not a good name for it as all addons (inclucing the whole default game UI) share the same global space and Blizz. tends to use "common" names for their stuffs) so DeathDiceStartGame maybe?