This likely means you are using a child theme and you’ve overridden the header.php file in your child theme. If this is the case you need to update the header.php file to include wp-site-blocks in the wrapper div.
The main thing is that you need to update:
<div id="wrapper" class="site">
To instead be:
<div id="wrapper" class="site wp-site-blocks">
The class wp-site-blocks was added because it’s a fixed class (meaning there is no option for developers to change it) in WordPress Core when Full Site Editing is enabled. As we look to build in more options to enable FSE we needed to move styles and settings to support this core class.
If you have header.php in your child theme then you are overriding the header.php file. Every file you put in your child theme you become responsible for. It’s now on you to update those files. The parent theme can’t update your child theme and so you should be very careful about which files you override in a child theme and also make sure you update those files anytime they update. Read more about why I don’t suggest you use a child theme at all.
I suggest you never ever override the header.php file. If you need to add scripts it’s best to do this through a function and not by taking control of a template.
Using functions means you don’t have to worry about template changes because your items are hooked into the right place.