To earn the Errand Boy trophy in Red Dead Redemption 2, you need to complete five companion item requests. There's two difficulties with this. The first is getting those requests to trigger, and the second is finding the requested items which are not marked on your map.
Here are the reported item requests, although some of them I was unable to trigger personally.
Jack -- The boy requested his Penny Dreadful book while taking him fishing in the mission "A Fisher of Men", so don't worry if this hasn't unlocked for you in camp. There are several books in the game, the nearest at the start of the game is in a hut east of Emerald Station labelled Osman Grove. Keep an eye out for a Fountain Pen in a drawer in here too -- see Mary Beth's request below.
Dutch -- One of the first item requests to appear for me in Chapter 2. Try to catch Dutch between 8am-8pm in the camp outside his tent, while standing and smoking a cigar. You'll find him a Pipe in a cabin above the "W" of WEST ELIZABETH on the map.
Tilly -- During Chapters 2 or 3, try and get Tilly into a Dominoes match and she will likely share the fact that she wants a necklace. You'll get Necklaces from looting homesteads and general robbery.
Abigail -- Reportedly she will ask you for a simple $5 at some point between 8am and 8pm in Chapter 2, though I couldn't get this to trigger.
Jack -- Chapter 2. After completing the first item request, Jack later approached me in the camp at daytime asking after a thimble. These can be retrieved from random dead bodies; it seems that the item only appears after the request has been made, or is at least much more likely to appear in loot.
Javier -- Chapter 2. See if you can catch Javier sitting and working on some throwing knives during the day. He will ask you for some Oleander Sage. It can be found in a few places, though PowerPyx
has a handy guide to a specific location that should also net you a random encounter.
Mary Beth -- Chapter 2. Catch Mary Beth during the day while she is writing in her book and you might be able to trigger a request for a new Fountain Pen. If you've headed to Osman Grove already for Jack and looted the place, you should hopefully have a Pen (unless you sold it).
Pearson -- Chapter 2. Once Poker is unlocked (after you have rescued the Reverend) you can start a game during the day and get Pearson to talk about his compass. You can find this in the Boathouse on the western edge of the Braithwaite Manor property, on a workbench by the entrance. It's technically outside of the no-fly zone if you haven't dealt with the Braithwaites yet, just don't ride too fast past the front gate or you will get shot at.
Sean -- Chapter 2. Allegedly asks you for some Kentucky Bourbon if you hang around camp, though I did not see this one trigger. Bourbon is incredibly common and can also be bought in a General Store.
Bill -- Chapters 2 - 4. Another one I wasn't personally able to trigger, but Bill just needs some Hair Pomade, again available commonly as loot and in the General Store.
Charles -- Chapters 2 - 4. Another one I wasn't personally able to trigger, but Charles wants some Moonshine. Head to a Fence to procure some. He may also ask for some Oleander so you can head to the same location above.
Hosea -- Chapters 2 - 4. Read the book on a barrel in Hosea's tent, some report he needs to be near the tent to trigger a necessary dialogue however I was able to trigger the request when he wasn't nearby -- Arthur makes a comment about finding another one for the old man. There's a hut called Hagen Orchards right under the O of BAYOU and south-west of Lakay -- you'll find the book in there.
Susan -- Chapters 2 - 4. Susan may ask for some Oregano during the day. I managed to trigger this in the Chapter 2 camp while encountering Susan near to Pearson's area. Oregano is very common in the Heartlands wilderness of Chapter 2.
Lenny -- Chapters 2 - 4. I managed to trigger this during a daytime encounter, though others report that he only makes this request at night. Lenny wants a Pocket Watch. They're fairly common in loot.
Pearson -- Chapters 2 - 4. The cook will apparently ask for a rabbit between 8am and lunchtime, though I can't personally verify this. Rabbits are everywhere, you've probably trodden on a few.
That should hopefully be enough to get things sorted for you. Just remember to start working on this in Chapter 2 and you should be fine.