Hi! What would you like to validate? If a user exists already? What is your scenario? Where and how do you get data to create a user? Via a form? Or from the workflow itself?

*) you can't validate anything inside the action execution. Only before or after (if action don't put workflow into suspended or terminated state).

 Regards,

 Tomasz

I am writing user on boarding process, SharePoint list or Form will be used to collect new users information. I am looking for the validation check that user does not exist .  If user already exist then I need to show appropriate message.

Well I don't know your level of expertise, but I would use a request to the SharePoint User Profile REST API each time a field in your form, dedicated for a login/ email, looses focus. If the call returns anything that would mean a user with the given login already exists. Based on that you can create a validation ride in your form and voilla

 Regards,

 Tomasz