As Julien wrote this is expected as you have an expression that starts
with a string:
("hello" whatever)
This will try to apply the string "hello" to the argument “whatever”. I
don’t know how to apply a string to anything, and neither does Scheme.
I would go with quasiquotation:
--8<---------------cut here---------------start------------->8---
(with-directory-excursion solution
((assoc-ref copy:%standard-phases 'install)
#:install-plan
`(("src" ,(string-append "include/" solution)
#:include-regexp ("\\.h$")))))
--8<---------------cut here---------------end--------------->8---
The expression following #:install-plan is quoted (so it’s inert data),
and we temporarily unquote to evaluate the “string-append” expression;
then we go straight back to “data mode”, so we don’t have to explicitly
quote “("\\.h$")”.