Cannot read property ‘nonce’ of undefined

A nonce error means that the admin-ajax.php was unable to complete. The nonce issue is most often caused by incorrect server settings or a plugin/theme conflict.

Can you try temporarily deactivating all your plugins that are not Ninja Forms add-ons and switching to a default WordPress theme like Twenty Twenty.
If you do not have a development environment for your site (this is best practice and highly recommended), you can use the Health Check plugin (https://www.ads-software.com/plugins/health-check/) to temporarily disable plugins and your theme for a single user session, instead of all users using your site.
If this solves your issue, then the problem was caused by an issue in one of those plugins or your theme. Reactivate them one at a time until the issue returns to determine which plugin is causing the conflict.

If you are still having this issue, can you reach out to our official support channel (ninjaforms.com/contact)