Kozo,
It looks like some part of Nix that isn't in /gnu/store still
refers to /gnu/store/pwcp…/bin/bash, which was garbage collected
(because nothing in /gnu/store referred to it).
Can you confirm that /gnu/store/pwcp…/bin/bash no longer exists?
If you like brute force, you can try grepping /etc, /var, $HOME
for the full /gnu/store/pwcp… string.
Kind regards,
T G-R