Hi,

As you are looking to do it from the user interface activate the debug mode and make sure that the module named(technical name) base_automation is installed in database.

Navigate to Settings -> Technical -> Automation -> Automated Action, here you can create a new record for this purpose, in the below example it is shown for preventing the duplicates in the field Email in crm.lead Model,

Give a name for the record let it be Prevent Duplicates in Lead, select the model as Lead/Opportunity, set trigger condition and in the action to do choose execute python code.

Python Code to add:

leads = model.search([('email_from', '=', record.email_from)])
if leads:
  raise Warning("Email Already Exists")

Click and save record, now when you create lead with duplicated email, it will raise warning like this,

Thanks