If you encounter this error, it means you are trying to send all of your transferrable funds out, without leaving a small transferable balance for the transaction fees. This is what the "insufficient balance" refers to: there's not enough balance left to pay the transaction costs.

The fee is shown on the transfer screen before you confirm the transfer. Deduct this amount from the total amount you're sending out, and your transfer will go through. Also, keep the existential deposit in mind when executing transfers.

You can see an example here: 

15.4 milliDOT = 0.0154 DOT

Alternatively, if you don't need the account anymore, you can also turn off the Keep Alive checks to transfer out all funds and reap the account.