Receiving a 403 response when trying to create Avatar IV videos

When making calls to https://api.heygen.com/v2/video/av4/generate, I receive a 403 response. I'm using the same x-api-key header that I used for other API calls which are successful, e.g., image upload, asset listing. I am currently trialing the HeyGen features with a trial account, which the quota response indicates has free videos remaining:

{'error': None, 'data': {'remaining_quota': 600, 'details': {'api': 600, 'avatar_iv_free_credit': 3, 'generative_element_free_concept_engine_credit': 3, 'b_roll_free_credit': 1, 'video_agent_free_video': 3}}}

From the docs, I don't see anything obvious that would block this call. Does anyone have tips on how to create Avatar IV videos via the API? Thank you!