Sometimes with exercises made in the past, Ninja Forms can have a couple issues.
Incompatible WordPress Versions
If when you try to install Ninja Forms, it says that it’s not compatible with your version of WordPress, then update WordPress from the Dashboard.
If the WordPress update does not actually finish because you have a slow internet connection, consult this document for a workaround.
Maintenance Mode
If you get a message about the form not displaying because it is in maintenance mode (which often happens with duplicated sites), you can turn that off in the plugin’s settings area. In the Dashboard, go Ninja Forms > Settings > Remove Maintenance Mode.