the endless os version of playonlinux is still in the alpha version and some stuff is buged out like the epic games launcher download please update it.
This is something to ask to the Endless OS distribution.
I looked at their website, it seems that they focus on Flatpak packages, so I think that (if you do not get answer from them) you should switch to Phoenicis (it is planed as Playonlinux v5): https://flathub.org/apps/details/org.phoenicis.playonlinux
