OpenAI no longer accepts prepaid cards for API usage even if it worked in the past.
This is a relatively new policy which seems to have affected many users especially those in other countries.
See Links Below for More Info:
https://community.openai.com/t/credit-card-declined-please-help/780000
https://community.openai.com/t/brand-new-credit-card-declined/299326
https://community.openai.com/t/your-card-has-been-declined-solution/731895