Seems that the liquidate buttons on the web interface(main page and porfolio page) don’t work during extended hours. Nor does the ‘Close all positions’ endpoint. I suppose this makes sense as only limit orders with limit price are allowable during extended hours - source
That said, it might be nice for these things to work during extended hours, as it’s helpful to have reliable stopgap solutions, at all times, without having to wonder whether they’ll work. At the very least, the web interface buttons could be disabled during extended hours (currently they just fail), so as not to confuse. And the close all positions endpoint could give a suitable response - I was getting 200 responses when nothing was actually happening.