vyos@r11-roll# set high-availability vrrp group FOO transition-script backup '/usr/bin/true'

  Warning: file /usr/bin/true is outside the "/config" directory
  It will not be automatically migrated to a new image on system update
  
  
  
  Invalid value
  Value validation failed
  Set failed

[edit]
vyos@r11-roll#

root@r11-roll:/home/vyos# /usr/libexec/vyos/validators/script /usr/bin/true
Warning: file /usr/bin/true is outside the "/config" directory
It will not be automatically migrated to a new image on system update
root@r11-roll:/home/vyos# 
root@r11-roll:/home/vyos# echo $?
1
root@r11-roll:/home/vyos#

vyos@r12-lts# sudo whereis true
true: /bin/true /usr/share/man/man1/true.1.gz
[edit]
vyos@r12-lts# set high-availability vrrp group FOO transition-script master '/bin/true'

  Warning: file /bin/true is outside the /config directory
  It will not be automatically migrated to a new image on system update

[edit]
vyos@r12-lts# compare 
+high-availability {
+    vrrp {
+        group FOO {
+            interface eth1
+            transition-script {
+                master /bin/true
+            }
+            virtual-address 192.0.2.45/24
+            vrid 120
+        }
+    }
+}
[edit]
vyos@r12-lts#