Ensure that all the podcasts listed in your CSV file are available in the Podcast Index database. The Podcast Index is a free, open, community-driven, and decentralized database of podcasts.
To check, visit Podcast Index and search for your podcast(s).
If a podcast you’re trying to add doesn’t appear in the search results, it is not yet listed in the database. To resolve this, go to Podcast Index's Add a Podcast page and submit the podcast’s RSS feed URL. Once added, you should be able to successfully upload CSV file and import your podcast(s).