How to make fields required in your form

I created a form for checking out equipment. What is the best way to alert a user when an item has not been checked back in at the time of their request? I attached a screen shot of the form and the pull down menu. Ideally I want a user to be alerted when their selection is unavailable.

The best approach you can use is by setting the filed to required. This way the form can never submit with the filed being empty.

Below is a guide on how you can set it up in your form.

Kindly do let us know if the solution above resolves the issues you had.