In my StellarMate system (last stable up to date) there is Xplanet already installed.
When in Kstars I center on Saturn and select 'View in Xplanet' I get no Xplanet window and Kstars says :Same happens with the Moon.
Contrarily, I get Xplanet screens for Jupyter, Venus and Sun but the images are spheres with no detail
Is there some problem with this application ? Maybe are there some image files I should download ?

xplanet-images packages installed.. let me check

In my system:
xplanet-images/stable,now 1.3.0-5.1 all [installed,automatic]

I observe the same problem in Ubuntu 20.04 LTS with KStars 3.5.2 stable.
In this case, KStars always opens the XPlanet window (gives no error as in Stellarmate) but the images for most planets are just spheres.
In the Xplanet application, in Ubuntu, I get correct images for Earth and Saturn but other planets, satellites and the Sun are just spheres of different color.