If you receive enrollment instructions for a particular course, you'll need to follow the instructions carefully. Don't rush through this process. In particular, you have to locate the right course (and the right batch number, if relevant) and enter the enrollment key exactly as given in the instructions. It's a good idea to copy the key from the instructions and paste it in the box where you have to enter it.
After entering the enrollment key, you can "unhide" it to see if it matches the key given to you. Make sure there are no extra spaces or other inessential characters.
If you can't join your course AND you're absolutely sure that you have tried enrolling in the right course with the right enrollment key, you may email the course facilitator to seek help. Please mention in your email the full name of the course you tried to join and the enrollment key you used.