Hi,
It's not a setting issue.
If you look at the HTML of the area where the button is supposed to be with the inspector tool of your browser, you'll see that it is there:
i.imgur.com/MLLM4S3.png
The problem comes from the CSS code you can see on the bottom right side of my screenshot.
In the file J4Test/templates/at_jewellery/css/custom.css you have the code:
.page-item.active .page-link, a.hikabtn.hikacart,
.slider-item .sppb-btn-success, .sp-simpleportfolio .sp-simpleportfolio-item .sp-simpleportfolio-info:hover,
.sp-simpleportfolio .sp-simpleportfolio-item .sp-simpleportfolio-overlay-wrapper .sp-simpleportfolio-overlay,
.btn, a.hikabtn.hikacart, .intro-item i {
background-color: var(--background-color);
}
In the settings of your template, you apparently have a setting for the color which is used there and it happens to be white.
If you remove the part a.hikabtn.hikacart, from there, you'll see the buttons again.