dnd 5e – Do I have to pick one of the listed types of dragon for my sorcerer’s Draconic Ancestry?

RAW: You can only select one of the listed options. Mechanically the problem of selecting some unknown dragon should be evident in the fact your class features wouldn’t have a table to refer to for Elemental Affinity.

You can always ask your DM for leniency or adjustments for the subclass though.