Sending Bitcoin from a pending transaction

Hive says I can't send a transaction before a previous transaction is confirmed, why is that?

The reason is that before you can send some Bitcoin to someone in a new transaction, you need to be sure first that you are allowed to spend those funds. If you try to spend Bitcoin and the network is not sure if they belong to you, your transaction might be rejected by the miners.

If you receive Bitcoin from someone, the transaction first appears as "pending" on your transaction list. This is because the Bitcoin network has received the sender's request to send you the funds, but the transaction still needs to be verified by miners. Until it's verified, there is still some (very small) risk that the transaction is cancelled and you won't actually receive the Bitcoin you were sent (this could happen if the person who sent you the coins is dishonest and tried to spend the same coins twice in two separate transactions).

Even though the chance of this happening is very small, the network might still reject your transaction if you try to spend those "unconfirmed" coins - so to prevent that, Hive might sometimes ask you to wait for the previous transaction to be confirmed first.