This error could be caused by one of two things.
The first is that you may not be taking the fees into consideration. Please make sure to calculate that the total funds you are sending, along with the fees is available in your wallet.
The second possibility is that Monero is waiting for change.
Monero has a balance and a spendable balance. And if you quite recently made a transaction, the Monero system is designed to wait for change first before you can make another transaction. Monero change requires 10 confirmations on the blockchain first, and until that completes, the change is locked and cannot be spent.
This is unfortunately a Monero software design and not our system that enforces this.
Once this clears and the change has confirmed, you will be able to make your next transaction.
This process does not have an exact time frame, but usually waiting 30 minutes between Monero transactions is a safe bet.