A method and apparatus are disclosed. In an example from the perspective of a first device, the first device is configured with a second set of slot offsets by a network. The first device receives, from the network, a second Downlink Control Information (DCI) scheduling a sidelink resource. The first device performs sidelink transmission, to a second device, on the sidelink resource. The first device monitors and/or receives a sidelink hybrid automatic repeat request-acknowledgement (SL HARQ-ACK) feedback from the second device in a fourth slot. The SL HARQ-ACK feedback is associated with the sidelink transmission. The first device derives, based on the fourth slot and a second slot offset value indicated by the second DCI, a third slot for transmitting a SL HARQ-ACK to the network. The second set of slot offsets includes the second slot offset value. The SL HARQ-ACK is set/derived/determined based on the received SL HARQ-ACK feedback.