OP is using electrum.
You can see the same error message here in the source code of electrum: https://github.com/spesmilo/electrum/blob/master/electrum/wallet.py#L2219

To get that error, you should right click on an unconfirmed transaction, select "Child pay for parent", set the "Fee for child" to an amount which makes the "output amount" smaller than the dust limit and click "Ok".