error TS: Property ‘dataset’ does not exists on type ‘Element’

The property dataset does not exist on type Element, but does exist on HTMLElement. You need to make sure it’s of this type before proceeding. It also works to adjust your for loop a bit:

for (const timelineEl of timelineEls) {
  if (timelineEl instanceof HTMLElement) {
    const progress = self.getProgress(timelineEl);
    const timelineKey = timelineEl.dataset.scrollTimeline;
    // ...
  }
}