Hello Shain,
In a Reactive App, you have the option to use Client-side validation to make sure certain inputs are not empty, in your example, you can ensure that the Name, City, and Address are filed.
If you still prefer Server-side validation you must disable the built-in validations option on the form button.

and create an Action to validate your data on the server, this should include a way to send any errors back .
I'm attaching an oml with an example of Server-side and client-side validations with custom errors messages.

Hope it helps!
Paulo Rosário