After making the base map, create a new layer and draw a vertical line with a white soft-edged brush. The width is up to you.

Enter the animation. In the first box, place the line at the far left of the basemap, copy the box and move the line to the far right of the basemap.

Go back to the first frame point frame by frame and set the number of frames yourself. If the effect is not good, you can change it again.

Then play it to see the effect. That's it.