Why carriers did not simply replace battleships before 1939

Was concentrating on battleships instead of more advanced carriers an obvious pre-war error?

No. Carriers were being developed, but battleships were proven and the future combat value of naval aviation had not yet been demonstrated at wartime scale.

Interwar planners had to fund capabilities that worked immediately while developing those that might become decisive. The Royal Navy already operated a large carrier force and continued investing in naval aviation. Carrier aviation in 1939, however, was not the mature system seen in 1944-45, and planners could not assume that no new defence would reduce aircraft effectiveness. Battleships remained a demonstrated capability for immediate imperial and fleet tasks. A mixed fleet was therefore a hedge against technological and operational uncertainty rather than simple resistance to aviation.