The Hobby plan allows 2 custom domains per service, not per account. Your "AI-engineering-website" service already has 2 custom domains (lithium-ai.in and www.lithium-ai.in), so it has reached that per-service limit. Your "lithium-app" service only has 1 custom domain, so you can still add another there. If you need more than 2 custom domains on a single service, upgrading to Pro would raise that limit to 20 per service.