The self serve offering only supports ads at the moment, and not sponsorships.
If you're not sure what the difference is, here's a recap:
These are ads from the many excellent brands that work with Acast’s shows to get their message into podcasts. These ads are typically crafted by the advertiser themselves, and may feature some signature “voice”, tone, or music associated with that brand.
These are typically direct endorsements of the brand/advertiser, and these campaigns are often read by the podcast creator herself. Owing to this personal connection, sponsorships are usually suitable where the podcast has a large listenership.