Please see Welcome to the forum - please reformat your message
Especially the bit of your message with italics and weird subscript letters.
That aside, however, I am still finding I cannot help you effectively because you are only showing tiny parts of your configuration.
The error message is pretty clear though:
when there are no instances of data.azurerm_monitor_action_group.default because its for_each is empty, then it follows that you cannot evaluate data.azurerm_monitor_action_group.default[each.key].