How do you generate an integrated address using a subaddress? I'm getting
Error: usage: integrated_address [payment ID]
It was decided that there should not be such a thing as an integrated subaddress. Only your main Monero address can become an integrated address (this functionality is retained for backwards compatibility).
The reason for this is that integrated addresses stand out on the blockchain. It is possible to see the difference between a payment sent to a standard Monero address vs an integrated address. This is because when an integrated address is used, an encrypted payment id appears in the "tx_extra" part of the transaction, and that makes the transaction stand out.
Since you can create as many subaddresses as you want, you can create an entire subaddress just for a particular sender. This means that you don't need to rely on a special payment id to identify the sender, because your wallet is able to correlate funds received to the subaddresses that you have given out.