Many problems with 5.6 Upgrade

Hi! Try to figure out what exactly is happening.
If the problem is with some script, it can be viewed in a console:

Using Your Browser to Diagnose JavaScript Errors

The temporary solutions to some compatibility issues are mentioned here:
https://github.com/wpmarketingteam/WP5.6Marcomms/blob/master/Questions%20and%20Answers/6_jquery.md
If something more serious, possibly server logs or WordPress debug logs will help:

Debugging in WordPress

If you will find the problem referred to plugin or theme file, contact its author for help.

What errors do you have in a browser’s console?
Did you install Enable jQuery Migrate Helper?

I cannot see this screenshot. Check, please, the link.

This files which are reported like missing in your installation are in place (I have checked a couple of them), so, something is going wrong during the process of retrieving them, it can be some mess with URLs.
Look a full URL to this files with 404 error (if you move a cursor on top of it in a console it will show up) and check ‘/wp-admin/options-general.php’ if ‘WordPress Address (URL)’ is the same as ‘Site Address (URL)’, possibly somewhere is www or different protocol (just a guess), the can be different, but usually, they need to be the same.
More info is here:
https://wordpress.org/support/article/moving-wordpress/#changing-your-domain-name-and-urls
Try to check if you have a correct .htaccess file as well:

htaccess

Try also to see if the Site Health Tool has some information as well (/wp-admin/site-health.php)