This error happens when you're trying to send less than the existential deposit (1 DOT on Polkadot and 0.00003333 KSM on Kusama) to an empty account.

If an account is empty, it is inactive. In order to be activated, at least the existential deposit (ED) needs to be sent to it. 

You can check the balance of the receiving account in the "Send" window:

So, if you get this error when trying to make a transfer, make sure you are sending at least 1 DOT to the recipient account, as it is inactive.

NOTE: Some exchanges list a minimum required deposit of less than 1 DOT. If your deposit address on the exchange is empty, you need to send at least 1 DOT to it, regardless of the minimum amount listed on the exchange's site.