> Hello,
> you might need to add the module.
> 
> (use-services-modules
>     ....
>     virtualization
>     ....)

Yes, this was the problem, thank you very much.
Gottfried



        (service libvirt-service-type
                      (libvirt-configuration
                       (unix-sock-group "libvirt")
                       (tls-port "16555"))))

       (modify-services %desktop-services
   (guix-service-type
-------------------------------------------------

and doing a:
sudo guix system reconfigure /etc/config.scm
it gave me this message:


Backtrace:
           18 (primitive-load "/home/gfp/.config/guix/current/bin/guix")
In guix/ui.scm:
    2263:7 17 (run-guix . _)
   2226:10 16 (run-guix-command _ . _)
In ice-9/boot-9.scm:
   1752:10 15 (with-exception-handler _ _ #:unwind? _ # _)
In guix/status.scm:
     835:3 14 (_)
     815:4 13 (call-with-status-report _ _)
In guix/scripts/system.scm:
    1276:4 12 (_)
In ice-9/boot-9.scm:
   1752:10 11 (with-exception-handler _ _ #:unwind? _ # _)
In guix/store.scm:
    656:37 10 (thunk)
    1295:8  9 (call-with-build-handler #<procedure 7fe023712ab0 at g…> …)
   2165:25  8 (run-with-store #<store-connection 256.99 7fe0205515a0> …)
In guix/scripts/system.scm:
     842:2  7 (_ _)
     719:8  6 (_ #<store-connection 256.99 7fe0205515a0>)
In gnu/system.scm:
   1276:19  5 (operating-system-derivation _)
    796:11  4 (operating-system-services #<<operating-system> kernel:…>)
    830:20  3 (services _)
In /etc/config.scm:
     38:21  2 (services #<<operating-system> kernel: #<package linux-…>)
In ice-9/boot-9.scm:
   1685:16  1 (raise-exception _ #:continuable? _)
   1685:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1685:16: In procedure raise-exception:
Fehler/mistake: libvirt-service-type: Nicht gebundene Variable/unbound
variable