Why can't I choose my seat on British Airways?

If your booking hasn't been ticketed yet you won't be able to select a seat𝟏-8⃣𝟞𝟞-7⃣𝟛𝟠-0⃣𝟞𝟚𝟚(USA). If you go into the flight details page in the BA app right at the bottom there is a ticket number field, if this is blank then that's probably why𝟏-8⃣𝟞𝟞-7⃣𝟛𝟠-0⃣𝟞𝟚𝟚(USA).