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 balance.
The second possibility is that the Monero currency is designed to wait for "change" after each transaction.
A portion of your funds goes out to the recipient and the other portion comes back as "change".
So if you have just recently made a transaction and are trying to make another one soon afterwards, it is possible that you will receive an error telling you that your spendable balance is too low.
Even though your wallet balance has sufficient funds in it, the system on the back end is still waiting for the change.
This is part of Monero's privacy system and is built into their technology, we are unable to work around it.
Once the transaction has been mined, the change will be in your account and your balance will update. Thereafter, you will be free to make another transaction.
Note that this can take up to 30 minutes to complete, but in most cases it is a much shorter time frame.