Mary Beth's errands are missions that must be completed in Red Dead Redemption 2 to get the "errand boy" trophy. One of these tasks is to search for a fountain pen, which will help you with detailed maps.
You can find the item in the Osman grove cabin, which is located in the East of the emerald ranch. It will be on the table next to the chair.
You can't find a pen on your own initiative. First you need to wait for the task from Mary Beth. She will give it to Arthur in the second Chapter of the game, in the camp of his gang, in the daytime between 8 and 20. the Main thing is not to miss it, otherwise you will not be able to get the achievement.
