Dear Léa
######@lea_grt2
Thank you for the screenshots, there is a prompt on the dashboard saying a PHP update is recommended.
Your site is running PHP version 7.2.34, PHP versions are now going above version 8.0.
Upgrading your PHP version is done with your Webhosting provider, you would need to contact them or possibly do this yourself from the webhosting website ( your account ). I would recommend doing this as i'm sure it will help in the future 🌝
This may be why the Customizer is not working as everything else seems like a fresh install of Lay Theme, sorry that you are experiencing this issue Léa.
Best wishes
Richard