This is an issue I have had since I got my Samsung Galaxy S8 a few months ago. I have searched repeatedly on the internet and not found a solution. Today, I worked it out. It is because your contacts are merged, ie phone contacts, SIM contacts, WhatsApp contacts for example. What you need to do is go into the contacts app and open the contact you want to add to favorites. On the top right hand corner click on the three vertical dots to bring up the menu; click on 'Manage linked contacts'. Delete SIM, WhatsApp and any other links, leaving only the phone contact. Once you have done this go back into your contact and the star is there. You can now add it to favorites.