Is there a way to add a notification or error message when a person does not submit the required information

Is there a way to add a notification or error message when a person doesn't submit the required information?  On the membership dues card, I have to select a membership, but most end users may not know to click the "+" to add it to the shopping cart or bag.  If possible, if I could add a notification that says click the "+" would be great.

You have to set the field as required. I see you already did this. Next, you can add the required information in the field description area (text below the field title/label).

You can also customize the Required Field message in the settings.

https://www.jotform.com/help/61-How-to-Change-Form-Warnings