When coins are being sent, there needs to be a Transaction Fee paid.
The Transaction Fee is shown above the Payment Address section and needs to be deducted from your total wallet value. It is paid in the currency that you wish to send.
For example, if the fee is 0.0005 BTC and you want to send 1 BTC, then you must leave 0.0005 BTC in your wallet to pay for the transaction, meaning to only send 0.9995 BTC.
If you do not consider the transaction fee when sending coins, then you will receive the insufficient funds error message.