It looks like you’ve accidentally moved ‘run:’ up into the previous line.
It should be:
## Any custom commands to run after building
run:
- exec: echo "Beginning of custom commands"
## If you want to set the 'From' email address for your first registration, uncomment and change:
## After getting the first signup email, re-comment the line. It only needs to run once.
#- exec: rails r "SiteSetting.notification_email='info@unconfigured.discourse.org'"
- exec: echo "End of custom commands"