Hello,

Sorry but this a WordPress core default problem.

Did you have any third party plugin installed? If yes try to disable it. You can also try to disable folie and activate the twentyseventeen theme and check if the problem persist.

Anyway please can you share with us your online link and wp creds (profile details) so I can check the issue closer.

Let me know.

Hello,

Sorry I have tried to check the issue but seems impossible 403 error.

I also tried on my local machine the classic editor and folie and there is no issue with the featured image.

I think the issue is caused by some third party plugin or you have disabled the featured image on the Screen Options:

https://imgur.com/fkITJQG

Let us know.

Regards!