As far as I can remember reading, bucklers were never historically worn on the arm. They were always held in the hand because of their use in dueling.

Of course, that doesn't mean you can't wear a buckler-sized shield (or something a little larger) strapped to your arm while retaining a free hand. We're not bound by historical veracity or anything. It's just not something that happened in the real world. It's a thing in D&D though (iirc there's a Pathdfinder class built around the idea of wielding a two-handed weapon while wearing a buckler). It's not impossible from a structural/anatomical/strategic standpoint.