Just open Spectacle and do a new screenshot using rectangular region. (window and full screen does work). Spectacle failed and asked me to open this bug report SOFTWARE/OS VERSIONS Linux/KDE Plasma: OpenSUSE Tumbleweed KDE Plasma Version: 5.21.0 KDE Frameworks Version: 5.79.0 Qt Version: 5.15.2 It was supposed to be fixed (Bug 377157)
When I run from konsole, I get: $ spectacle --region Error calling KWin DBus interface: "org.kde.kwin.Screenshot.Error.ScreenMissing" "Screen not found" QPixmap::scaled: Pixmap is a null pixmap
*** This bug has been marked as a duplicate of bug 432260 ***