I hand-sculpted bases for my chess set before realizing that I wanted them to be very identical. So I wanted to “cut” the hand-sculpted base off of the figure and replace it with a programmatic one. Except I kept getting this error using box trim — Not supported in dynamic topology mode.

Switch to a drawing brush and uncheck Dynamic Topology box

Then switch back to box trim and it actually trims

And then, of course, you need to remember to turn dynamic topography back on for the drawing tools to draw.