Hello,
Sorry for the inconvenience but actually, the new Widget style of the WP shows Warnings even with the Default WP Theme too and doesn’t work well.
So, we would recommend you to install the “Classic Widgets” plugin on your Site and then try using the Widgets area as per your choice and check back the Widgets again.
Here is the Plugin URL: https://wordpress.org/plugins/classic-widgets/
If somehow the issue still persists, then please provide WP-ADMIN Login details of your Site so that we can proceed further with that and look after this.
Best Regards