You can't delete the Nickname Field when using the Drag and Drop edit mode of the forms, you could only autogenerate it.
You could hide it, for example inside a Conditional Block (that never triggers to visible), or wrapping in HTML blocks that hide the content (class="hidden")
In Expert Mode, you could delete the source code.
Now, on an Edit Form, the User Name is already set, and can't be changed by WordPress Default Behaviour, so it is also not possible to autogenerate it.
On a Form that creates users, though, it makes sense and is possible.
If autogenerated, it would then generate the Username and Nickname in a bit unexpected way:
Using the email
So if the user completed the email field with "email@domain.com", then the Username and Nickname, if autogenerated in the Form, will use "email"
To manipulate that value further, you'd have to apply some custom code to the cred_save_data() hook:
https://toolset.com/documentation/programmer-reference/cred-api/#cred_save_data
However, it'll be tricky to update the Username, because WordPress itself does not allow this.
The method to use https://developer.wordpress.org/reference/functions/wp_update_user/ would be the logical solution but it won't update the username, see https://wordpress.stackexchange.com/questions/103504/how-to-programatically-change-username-user-login
You'd have to resort to direct database interaction as shown on the Stackoverflow ticket
It is eventually easier to populate the (hidden) username field with some custom JS on the fly before submitting the form, by grabbing the data from the user first and last name inputs.
Are you comfortable writing and maintaining either PHP or JS?