Found this on main Ninja Forms site
“We are working on the following known issues:
Our developers are currently investigating a conflict with an Elementor script which is currently affecting the ability to edit forms. Our engineering team is investigating the conflict and reaching out to the Elementor team for insights.
To alleviate the issue short-term, please temporarily roll WordPress back to version 5.5.3.
Alternatively, you can try using the “Enable jQuery Migrate Helper” plugin:
https://www.ads-software.com/plugins/enable-jquery-migrate-helper/
After installing this, navigate to Tools > jQuery Migrate and set the jQuery version to “Legacy 1.12.4-wp”.”
So I guess there may be a fix soon?