Shin is right, there's no dashes as a Line formatting option in Tableau.
It doesn't necessary mean that one couldn't have dashed lines in Tableau.
They could be there, but it would not be just lines connecting dots,
but rather complex objects looking like.
Please find these two (complimentary) articles on a subject:
My advice: don't do it (even if you could).
Use (complimentary) colours instead.
Thank you for the support always.
Shin, you're always welcome.