They are two completely separate balances, and mixing them up is the most common confusion in the app.
Coins are demo money for the simulator. You earn them from lessons, streaks and daily rewards. They fund your practice trades. They have no cash value and cannot be bought, sold, transferred or withdrawn.
Credits power the AI features — Fin's explanations and the explainer tools. Some come with your subscription; some can be purchased. They are consumed when you generate an explanation.
The short version: coins buy pretend stocks, credits buy real explanations. The app shows them in different colours for exactly this reason.
Neither has cash value and neither is refundable. If an AI request fails, the credit for it is returned automatically.