You didn't describe what you're doing so I'm assuming this is a form region and you want the message to appear on change of the page item. Here is an example with a form on the EMP/DEPT sample data. Functionality is that on change of the item P59_ENAME, a dynamic action fires and shows a javascript alert if the ename already exists. In my case this is page 59. I tested this on apex 21.2

1. Create a form on table EMP
2. Create an additional page item of type "Hidden" named P59_ENAME_EXISTS
3. Create a dynamic action on the item P59_ENAME

3. Add a first true action to execute the pl/sql code. This will set a value for the page item P59_ENAME_EXISTS (Y if ename exists, N if it does not exist). Note the "items to submit" and "items to return" values - those are very important.

4. Add a second true action to show the javascript alert if ename already exists. Note the client side condition.

5. Add a 3rd action to reset the value of page item P59_ENAME_EXISTS

Run the form, change the ENAME to an existing value and click in another field on the page. You'll get the popup.

Triggers or Automations cannot be used to achieve this functionality.