Dashed line on adobe fresco?

Fresco doesn't have a dash line, but you can create your own brush, which could be dashed, using Photoshop and save that brush to a library, library that you can open and use in Fresco.
You can also use an already dashed brush, for example Kyle Webster has some dashed lines for shading work in his screentone series (special effects brushes) that while not what you are looking for, prove that it's possible.
See also the "dotty" ones from the letterer series (see below)

If you create such brushes, be sure to remove any pressure and tilt control on them, so the effect is even (unless you want it with variations).

If you need guidance to create brushes, let me know...

Fresco does not have an option to turn a brush into a dashed line.

You would have to continuously lift and drop the pen to achieve that sort of result. Unless you can find and install a specific dashed line pen I think you would have trouble creating the effect evenly.