When you try to send funds out of your account on Polkadot-JS UI and get the error message 'balances.KeepAlive', the system protects you from potentially losing your funds.

This happens when you are trying to send all of your DOT or KSM out (or enough to drop your remaining balance below 1 DOT or 0.000333333 KSM), but the "keep alive" protection is enabled, which is the default. 

Accounts with a balance of less than 1 DOT will be deactivated on the Polkadot network (0.000333333 KSM on Kusama), which means that all remaining DOT in that account will be burned (irrevocably lost). So, the "keep alive" protection is enabled by default to prevent this from happening accidentally.

The reason for this is the required Existential Deposit (ED) of 1 DOT on the Polkadot network, which you can learn more about here. 

Solution

If you want to send all of your funds out, you can switch the "keep-alive check" off:

And toggle the button to transfer out the full amount:

After the transfer, your account will be deactivated, although you can always reactivate it in the future with a deposit of more than the existential deposit.

If you want to keep your account active, reduce the amount you are sending out and make sure you keep a balance of at least 1 DOT (or 0.000333333 KSM) after applying fees. 

If you want to see the error and solution in action, check the video below at timestamp 3:14: