Some course sections use a registration functionality called reserved seating. Reserved Seating allows certain departments the ability to maximize their resources in providing the most seats for the most students throughout the University.
If you reached this link from a registration error message you have not been able to register for a specific lab or section. The university is currently utilizing a registration feature for this lab/course called Reserve Seating. High demand courses such as the one you've tried to register for often fill up early and not always every student registering for the course must have that class during the current semester to graduate in a timely way. Please try registering for other sections of that course before contacting your advisor.
You may be in a major or early in their program of study where this course is necessary for graduation but may be taken in a later semester than the current one. The University understands that is important to offer courses in a timely fashion so you can stay on track for graduation and we are doing our best to offer additional seats and summer offerings of high demand courses.
It is very important to keep in mind that often during the registration process, seats become open as other students change their minds about courses, change majors, must retake an earlier course before the one they are currently registered, switch sections and many other reasons that lead to students dropping a course. It's very important that you continue to check for open seats in the weeks leading up to the next semester and during Drop/Add.
Years of experience in seats management has shown that most students are able to get the course they need, when they need it but it does happen sometimes as late as the week the semester begins because that is the week during which students may adjust their schedules through the drop/add process.
If you still have concerns or would like further assistance reach out to your advisor.