FYI, not sure if this is a bug or not.
I attempted purchasing a hobgoblin ancestry for a character just now. I don't have any Playtest points, so I didn't change that dropdown menu. I selected my character in the dropdown menu for AcP and clicked Purchase, but the page just refreshed with no change.
I tried again, this time selecting my character in both the Playtest and AcP dropdown menus. Now when I clicked Purchase, it worked.