I think it may be just a matter of waiting. This note on the cache page from GC HQ seems to suggest that:
“Thank you for celebrating cache quality and cache creation with us.
Due to the large volume of Creation Celebration Events submitted to Geocaching HQ, approved Creation Celebrations will have to receive their souvenir awarding qualities in batches.
This Event was not submitted to Geocaching HQ before the cut-off date for the second batch - Monday, April 1st or only after it had taken place. It was not captured by the regular souvenir assignments and Geocachers logging this Event will hence currently not be awarded the Creation Celebration souvenir. All Events submitted after the above date will be awarded the souvenir retroactively after the end of Cache Carnival.
How do you get your souvenir?
Log this Event right away and wait until the end of Cache Carnival. Missing souvenirs will be awarded retroactively via a script.
Thank you for your understanding and happy celebrating.
Cheers from Seattle”