When I l click on 1) add a dynamic page 2) Item Page (next) 3) Remove {Title} 4) Click on Add Field button and choose a field, I get the following error message: The URL prefix is already taken; choose something new. Ok, what am I missing???
As the pop up error message informs you that your chosen url prefix is already taken.
It basically is already in use so you will need to create a new one.
The setup of a dynamic item page is not code related and therefore you should be going through Wix Support for more help with this issue.
https://support.wix.com/en/about-wix/contacting-wix-support
https://support.wix.com/en/article/adding-and-setting-up-a-dynamic-item-page
Corvid by Wix | How to Create a Dynamic Item Page and Index Page - Youtube walkthrough
Note: Every page on your site has a unique URL. If your dynamic page URL isn’t unique, you will receive an error message.
https://support.wix.com/en/article/creating-a-unique-dynamic-page-url
What do you mean url prefix??