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