If you encounter this error it means that you are trying to send all of your transferrable funds out, without leaving a small balance for the transaction fees. This is what the "insufficient balance" refers to: there's not enough balance left to pay the transaction costs from.
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.
You can see an example here:
15.4 milliDOT = 0.0154 DOT
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