Error:400 Bad Request

[yuri]

To disable the "required" parameter, follow: Administration > Entity Manager > {Your Entity Type} > Fields > name. Uncheck "Required" and save.

You can also utilize formula to set the 'name' field with a value from some another field. It's recommended to have the 'name' field not empty.

[shalmaxb]

This refers to the default field in every espoCRM entity withe the neme "name". You will see this field in every entity and in every new custom entity. You can rename the label, but not the name itself. It is mandatory, becaus it is the default field, which links to the detail view.
In your case it seems, that you took this field off the layout and substituted by another (own) field. So, as it is mandatory, there is no value in this field, which throws this error.
If you do not want the name field, you may give it a default value and hide it from the layout. This way it would have a value but you would not have to use it.
The necessary link to the detal view you may configure in the layout manager by giving another field the link.

[yakup aycıl]

I created a field called prospective customers. But why is it giving this error while recording?