Hi @TharwatSamy, glad to hear you will be publishing your first course soon!
Some instructors choose to launch their courses as free to generate a following, capture reviews, and get feedback, then switch to a paid course at a later date. Other instructors prefer to create free coupons instead and provide them to students in order to achieve this. It's important to keep in mind, free courses cannot exceed 2 hours in video length and will lose all premium learner features which includes a certificate of completion and Q&A. On the other hand, paid courses do not have course length limitations and cannot be offered for free elsewhere (for example, YouTube, your own site, other platforms, etc.).
You can publish your course for free and change its price at any time you want, however, you can only switch from free to paid one time. I recommend reading the following article to learn more: Free Courses: What Should Instructors Know?