If you are visiting this page I guess you probably just upgraded your old WordPress website to the latest 5.8.x version (5.8.0 or 5.8.1). Then checked your Widgets page and saw a screen like this:
Am I right?
Don’t worry your website is not broken. WordPress neither broken as well. Most likely your theme is not updated to be in sync with all latest WordPress features – WP is moving towards blocks instead of widgets.
You will see such message if you check your debug.log:
wp_enqueue_script() was called <strong>incorrectly</strong>. "wp-editor" script should not be enqueued together with the new widgets editor (wp-edit-widgets or wp-customize-widgets). Please see <a href="https://wordpress.org/support/article/debugging-in-wordpress/">Debugging in WordPress</a> for more information. (This message was added in version 5.8.0.)
You shouldn’t worry if you are using frequently updated WordPress theme. Inform your theme developers and they should fix this in future versions.
It is not an option if you are using WP theme which is not supported by authors any more.
But again don’t worry about it. It is easy fixable as well. WordPress developers foreseen such cases and they developed Classic Widgets plugin
Install this plugin and you will see old fashion Widgets screen without any errors.
Let me know in comments if this was helpful. Or may be you found another solution for this issue. Let us know about it as well.