Yes, you can surely apply for the Common Law Admission Test while appearing for the board exams of class 12 for admission to UG courses. The eligibility criteria for CLAT states that the candidates who have passed their class12th Board Examination or The Senior Secondary School Examination can apply for the exam. In addition to it, the aspirants have to secure a minimum of 45% marks in the qualifying exam for General Category and for SC and ST candidates, they need to secure 40% marks in qualifying exam. However, not only the students who have passed their class 12 examination can apply but also the candidates who are appearing in their qualifying exam can appear. In the case of admission to UG courses, this qualifying exam refers to Class 12 examination and hence you have all the rights to appear for it and there is no problem in applying for the exam but you also have to ensure that you fulfill the educational (and also other) eligibility criteria for the exam. If you want to know more about CLAT 2020, you can refer this link

https://law.careers360.com/articles/clat-eligibility-criteria