Gemini API 429 quota error on Free Tier

Hi everyone,
I am using Gemini API with a Free Tier project and the gemini-2.5-flash model.
Every generateContent request returns:
429 ResourceExhausted:
“You exceeded your current quota, please check your plan and billing details.”
The project is marked as Restricted, while the API and model are available. The Usage page records the requests, but every request fails.
How can I check whether the Free Tier generateContent quota is set to zero? Could this be related to account or regional eligibility?