How to have my posts centered

It should work in all single posts.

Can you guide me to a page that's not working?

Sure, https://getmoresteps.com/why-walking-is-good-for-health/

Can you try uploading a wider image?

Thanks, yes it works when I change to a bigger picture.
Is there a way I can have the title of my post in the feature image?

Then you will need a header element:
https://docs.generatepress.com/article/header-element-overview/

See example here:
https://docs.generatepress.com/article/page-hero-examples/#example-2

And step by step guide:
https://docs.generatepress.com/article/how-to-create-a-page-hero/

Please open a new topic if you need further help with this.

I'm going to mark this one as resolved.

Thank you very much!

No problem :)