Thank you for the direct actionable feedback, we will improve that messaging.
Regarding debugging your specific problem, when an API is attempted to be added, the local process attempts a 1-token request to the cheapest model with the GPT platform (in your case, gpt-4o-mini on OpenAI) to verify that the key works. Though, if the account has no balance, this request may fail even though it costs a fraction of a fraction of a penny (though anything that fails that request will cause the API key to be considered invalid).
Regarding debugging your specific problem, when an API is attempted to be added, the local process attempts a 1-token request to the cheapest model with the GPT platform (in your case, gpt-4o-mini on OpenAI) to verify that the key works. Though, if the account has no balance, this request may fail even though it costs a fraction of a fraction of a penny (though anything that fails that request will cause the API key to be considered invalid).