There is a known issue in Eltron theme, that Slider does not appear when the Elementor editor is on.
As a work around, please use below method to edit the slider.
Please move your mouse to the place the slider is and the row will be selected, by clicking it you can edit the slider from the left as usual, as seen in the below image.
https://share.getcloudapp.com/NQu1kEBE
