Would you try to look on the Theme Settings or Header section?
You are using a $180 theme and you should have the support from the provider of it.
You should try to contact them for help or checking the theme document for the sticky header
I checked "motion" default theme already has header sticky. Can you please re-check and let me know?
If you can not find it, you can add me as a staff, I'll check it for you (mail: email@example.com)