Fig.: you can get from people only what they are able or willing to give; trying to do an impossible task

After going bankrupt the poor man was forever being squeezed by creditors. Evidently they don't know that you can't get blood out of a turnip

Dopo la bancarotta il pover'uomo disse che ormai era stato spremuto come un limone e non ce n'era più per nessuno. Dopotutto non si può cavare sangue da una rapa!

It looks to me that the new government is concentrated on ways and means to get more blood out of the taxpayer-turnip

Mi sembra che il nuovo governo si concentri soprattutto su come cavare ancora più sangue dai poveri contribuenti-rapa

You cannot, obviously, extract something that is not there to begin with. In the turnip there is absolutely nothing that can remind you of blood