Sometimes you cannot enrol in a unit yet because you have not yet officially met the enrolment rules. Most often this is just because your results for the prior unit(s) have not yet been published. For more information about final results, see: When and how will I get my final results?
There are a number of options to check your unit enrolment requirements e.g. to see if your unit has a prerequisite (sometimes also called a pre-req or pre-rec), co-requisite or other restriction.
Progressive placement units can only be self-enrolled if you have officially completed the placement unit prior. Otherwise you will be manually enrolled into your next placement unit by the enrolment team subject to you having hit the progress point for the previous unit.
First, check your Course Handbook. If you still need more information or you find your Course Handbook at all unclear, please contact your Course Coordinators.
You can also hover your mouse over the unit code in your Course Plan in Paradigm. The pop-up will tell you your current status for the unit, and any pre-requisite units or other course rules. See: Course Plan in Paradigm