Hi, @henry . You don’t mention where you’re having the problem, so I’ll start from scratch.
If you’re accustomed to the Wix Editor, you may be having trouble finding the mechanism for adding a lightbox in the the first place. In Editor X, you’ll find it in the Add Elements panel of the left sidebar.
Click the Add Elements icon in the toolbar (1), choose layout tools (2), pick lightboxes (3), and then choose one of the predefined lightbox options (4), which you can edit to match your needs.
Once you’ve got your lightbox, you can edit it and adjust the settings. You mention that you want to call the lightbox from a button, so you’ll need to immediately change one of the default settings (see below).
By default, the lightbox is set to display automatically on pages. That’s not what you want, so you need to select the lightbox and, in the element toolbar, click 'Set Triggers (1) and then change the Lightbox Page Settings – Automatically Display on Pages – to ‘No’ (2).
At this point, you’ve got a working lightbox. You can edit it for your specific needs, and you can edit buttons to call it.