I would make a bold assumption that you (or someone, or an app) has made changes to your theme and remove the code that adds the <title> element. This is 100% not normal and every theme from the Theme Store will have the correct code in place.
Have you edited the code?
You may have to share some code if you're hoping for the theme members to help you troubleshoot. For your theme, the <title> tag is within the theme.liquid file so seeing some code from the top of that would help.