What does error 78 mean on a Daikin air conditioner?

Daikin

Daikin air conditioner error 78

System No. 2 Malfunction of fan inter lock

General meaning for the brand's models, unless a model's own manual says otherwise.

Indoor or outdoor fan motor error: causes and checks
When the unit reports a fan error: check the mechanics, connectors, supply and speed feedback before replacing the motor or the board.