Can I use aria-hidden on DIV

It looks like you cannot add aria-hidde="true" to an element that has a focus within it. Is there a descendant element in the lightbox that is clickable?

The following is from Mozilla Developer Documentation:

Description

Adding aria-hidden="true" to an element removes that element and all of its children from the accessibility tree. This can improve the experience for assistive technology users by hiding:

• purely decorative content, such as icons or images
• duplicated content, such as repeated text
• offscreen or collapsed content, such as menus

According to the fourth rule of ARIA, aria-hidden="true" should not be used on a focusable element. Additionally, since this attribute is inherited by an element's children, it should not be added onto the parent or ancestor of a focusable element.
Using aria-hidden="false" will not re-expose the element to assistive technology if any of its parents specify aria-hidden="true".

I'm not sure what #lightbox does as it is handled by the Appointment Pro theme.

I do see that it is hidden to sighted users with display: none; but don't know if this changes. How do I handle this?

I didn't  know that the Lightbox is popup, how do you activate it?

I don't know the details of your theme, but that empty link is probably the trigger. It needs to be an anchor that works as the method to open the popup. That's why there can't be an actual link in there. Everything is happening in a js file. The code in that file is referencing that ID: #lightbox. There is probably some initialization code in the head of the document as well, that is referencing that ID.