You must have an "All Access" iHeartRadio subscription to be able to create playlists in your iHeartRadio account. See iHearRadio pricing here: https://www.iheart.com/upgrade/
If you have no iHeartRadio subscription or a "Plus" subscription, you will not be allowed to create playlist in iHeartRadio. That's why on Soundiiz side, an error message is also displayed.
Be sure to subscribe to "All Access" plan on iHeartRadio before trying to migrate your playlists from another platform to iHeartRadio.
Also, be aware there's a limit of playlists you can have in an iHeartRadio account. Once this limit reached, you will not be able to create more playlists.