When selecting courses, use your registration tools (degree matrix/academic roadmap/transfer evaluation, College Catalog, and unofficial transcript).
You may find that a course you need or want to take in a specific term is not available either because all of the seats are taken or because it is not currently being offered. While it may be frustrating not to get your first choice, you can make the process of selecting an alternative a little easier by knowing how to troubleshoot.
If you believe you need to take a specific course in a specific term and you see that it is not offered or is full, contact your advisor! Your advisor can help you determine if you can wait to take the class, and if not, they can help you create an alternate plan to take the course when you need it.