You can find that option through Admin Dashboard > Appearance > Customize > Blog > Single Posts (General) > Reading Progress Bar.
1. Reading Progress Bar: Toggle option to enable or disable the reading progress bar.
2. Bar Position: Control the position of reading progress bar (top or bottom)
3. Bar Height: Control the reading progress bar height.
4. Include Comments Area: The progress bar fill 100% when the last comment is shown in the browser view.
This feature has styling options as well. You can find these options through Admin Dashboard > Appearance > Customize > Blog > Single Posts (Style) > Reading Progress Bar.
1. Background Color: Controls the background color of reading progress bar wrapper.
2. Foreground Color: Controls the color of the foreground bar.