I haven't tested but at first glance I would say that the target of the mapped-device and the root filesystem device don't match: one is "root-part" and the other is (file-system-label "root-fs").
According to the example in the manual ("(guix) Using the Configuration
System"), you should replace
(file-system-label "root-fs")
with
"root-part"
Also watch out with those `cons', you should use `cons*' or `list' if someday
you'd like to add more than one element.
--
Pierre Neidhardt
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