Yes, if you could not qualify two subjects in CBSE 12th board exams, then you can appear for improvement exams as a private candidate. However, if you want to take admission into some college this year itself and if it requires 12th board marks, then you also have an option to qualify the failed subjects this year itself. You can do so by registering for NIOS on demand examinations.

After registration, you will get an option to select the date and center as per the availability and your choice. You may appear for them and qualify the failed subjects.

Hope it helps.