Why do I need to have at least 0.03SOL?

For the first transaction, ≈0.024 SOL is required (creating a transaction wallet), so Serum Orderbook requires a minimum holding of 0.03 SOL.

In order to trade, you will need to deposit some SOL to your wallet.

Due to Serum's design, there is need to open a trading account for the pair when you first trade it.

So, the “first trade” fee is ≈0.024 SOL