You don't seem to be incrementing the count variable in setInterval nor invoking the carousel function like you do, when the next or previous button is pressed. Update your setInterval to increment the counter and invoke the carousel function.

setInterval(()=>{
count++ //increment the counter
carousel() //call the function
}, 3000);

Your index file should then look something like this:

const slides = document.querySelectorAll(".slide");
const prevBtn = document.querySelector(".prevBtn");
const nextBtn = document.querySelector(".nextBtn");

// Inline callback function
// forEach(function(element, index, array){ ... })
slides.forEach(function (slide, index) {
  slide.style.left = `${index * 100}%`;

  // console.log(slide);
  // console.log(index);
  // console.log(array);
});

let count = 0;
nextBtn.addEventListener("click", function () {
  count++;
  carousel();
});

prevBtn.addEventListener("click", function () {
  count--;
  carousel();
});

function carousel() {
  slides.forEach(function (slide) {
    if (count > slides.length - 1) {
      count = 0;
    }
    if (count < 0) {
      count = slides.length - 1;
    }

    slide.style.transform = `translateX(-${count * 100}%)`;
    // console.log(slides.length);
    // console.log(slide);
    // console.log(count);
  });
}

setInterval(()=>{
  count++
  carousel()
}, 3000);