Why does the slider "not see" the photo?

Question

css

Why does the slider not see the photo? ((((((((((( I've been sitting for 5 hours and tried 4 sliders...
I downloaded the ready-made code for the slider
HTML code

</div>
    <ul class="cover-slider">
        <li class="cover-slider__slide">
            <span class="hide">Alt Tag</span>
        </li>
        <li class="cover-slider__slide">
            <span class="hide">Alt Tag</span>
        </li>
        <li class="cover-slider__slide">
            <span class="hide">Alt Tag</span>
        </li>
        <li class="cover-slider__slide">
            <span class="hide">Alt Tag</span>
        </li>
    </ul>
</div>

CSS code

.cover-slider__slide:nth-child(1) {
  background-image: url("https://unsplash.it/640/480");
}
.cover-slider__slide:nth-child(2) {
  background-image: url("https://unsplash.it/641/480");
}
.cover-slider__slide:nth-child(3) {
  background-image: url("https://unsplash.it/641/481");
}
.cover-slider__slide:nth-child(4) {
  background-image: url("https://unsplash.it/643/482");
}

I change the link to another photo
https://unsplash.it/640/480 =>> https://unsplash.com/photos/nvtJoYiIM2Q
or
https://unsplash.it/640/480 =>> background-image: url ("img/1.png");
DOESN'T SEE..... (((only an empty white window shows....
background-size: cover;
background-position: center;
repeat: no-repeat;
I also added these 3 lines, it still doesn't work... And so 4 different sliders...
What could be the reason?

Answer 1

This is a link to a page with a photo, not a photo
https://unsplash.com/photos/nvtJoYiIM2Q
Here is a link to the picture
https://images.unsplash.com/photo-1494719019271-7b...

Answer 2

And without the slider are the photos visible?