Property or indexer of type RepeatedField cannot be assigned to — it is read only

I found an answer to my own question I asked above.

        List<uint> lockerIds = new List<uint>();

        ProtoPacket protoPacketResponse = new ProtoPacket 
        {               
            AvailabilityOfLockersResp = new AvailabilityOfLockersResp { NumberOfAvailableLockers = (uint)lockerIds.Count() }//LockerIds = lockerIds,
        };

Outside of new instance, I have assigned the value to the LockerIds like below,

protoPacketResponse.AvailabilityOfLockersResp.LockerIds.AddRange(lockerIds);