tlp-shepherd-service tries to start bin/tlp, but looks like the binary has relocated to sbin/tlp somewhere between 1.5 and 1.8 (updated in 0b8ceaa1cd26a73059a69d8df50f5de3775eaf37).

diff --git a/gnu/services/pm.scm b/gnu/services/pm.scm
index 64b64e01fb..9b8066a38f 100644
--- a/gnu/services/pm.scm
+++ b/gnu/services/pm.scm
@@ -448,7 +448,7 @@ (define-configuration tlp-configuration

 (define (tlp-shepherd-service config)
   (let* ((tlp-bin (file-append
-                   (tlp-configuration-tlp config) "/bin/tlp"))
+                   (tlp-configuration-tlp config) "/sbin/tlp"))
          (tlp-action (lambda args
                        #~(lambda _
                            (zero? (system* #$tlp-bin #$@args))))))