I've never had this issue creating a form either, so let's try some diagnostic questions.

First, let's understand what a 403 error means. The HTTP 403 Forbidden response status code indicates that the server understands the request but refuses to authorize it. With the 403 Forbidden status code re-authenticating makes no difference. The access is permanently forbidden and usually tied to an application's logic, such as insufficient rights to a resource, but which application's logic, WordPress or Formidable?

1. Have you tried resaving the WordPress permalinks?
2. Are you certain you have the right server permissions? YOu may need to have your host's support team help with this.
3. Are you using any security plugins like WordFence perhaps? Security plugins can block form actions unless exceptions are applied.
4. Are the entries being saved to the database? If they are, then the problem may be related to a redirect.
5. After an entry is saved, are you redirecting to another page like a thank you or something else, or are you displaying the default message?
6. If all else fails, try walking through WordPress conflict checking. Disable all plugins. Does the error go away? If so, enable each plugin one at a time and save a new entry until you find the conflict.
7. Enable WordPress debug and look for errors in debug.log.
8. Check the server PHP error logs for errors. PHP error logs are usually found in the site's root directory or wp-admin although they can be in any subdirectory when a PHP error occurs.
9. Are you using the latest versions of PHP, WordPress, and all plugins?
10. Check the form permissions in Formidable's global settings and on the form's permissions page.

Ok thanks let me try all the checks you listed 

Hello please i check my site and everything seems to be working fine then i tried some few things with the formidable. I started imputing the field one at a time then update and noticed when i tried to use the drop down field that’s when i start to get that 404 redirect error. Is there something i am not doing right with the drop down please assist me urgently. 

My steps :
- i click on the drop down the then rename the field label
- set it to required 
- give it a class of frm_half
- the drop down option name the first Male and second Female 
then i click on update. 
But when i do that i get the error please help me 

The issue is that i can’t use the drop down… and i would like to use it for an input how do i go about it please 

Got, exactly the same problem...latest FF, latest WP, PHP 7.4. Only dropdown is giving the 403. Weird stuff.

Problem solved by my webhotel provider, they had to update their ModSecurity rules and that was causing the error.