Comment 2 for bug 1888539

Please recompile the OpenLoops process library with

./openloops libinstall <processes> compile_extra=1

or more generally create a file in your OpenLoops directory named openloops.cfg and put in the lines

[OpenLoops]
compile_extra=1

This will compile the real radiation tree amplitudes, as stated here https://openloops.hepforge.org/download.html