If you encounter the error 'balances.InsufficientBalance', 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.

You can see an example here:

15.7397 milliDOT = 0.0157397 DOT

However, if you want to send all your funds out, you can indicate it from the Polkadot-JS UI and leave the system to deal with the transaction. Follow the steps described in the article "Polkadot-JS UI: How to Send All of Your Funds Out of Your Account."

If you are more of a visual learner, check the video about how to transfer funds out of your account at the mark 3:01: