If you follow a course's redemption link in your Account and get a "Sold Out" or "Invalid Code" error, you most likely already redeemed it. Once you redeem a course once, it will be in a separate account with the website that is hosting the course.
For example: If you purchased a course on Udemy, you only redeem it once with us. After that, it will be in your Udemy account forever. Log into your account at Udemy.com and click on the "My Courses" link at the top right to see all your Udemy courses.
Tip: Sometimes customers accidentally create 2 Udemy accounts. If you know you've redeemed the course, but you don't see it in your Udemy account, try logging in with another email address.
If you have any questions, feel free to contact Customer Support.