If you run a virtual machine (VM) with the KDE desktop, your host clipboard will not be shared with VM guest. That means text you copy inside your host can’t be pasted into the VM, and text you copy inside the VM can’t be pasted into the host.
Bugzilla report: #2016563
One possible workaround is to log out from the current KDE session, select “Desktop Session: Plasma (X11)” at the very bottom of the login screen, and then login again.
With an X11 session, this bug will not occur.