If you want to save Dashbaord into PDF, You have to follow this steps.
1. Click on File-
2. Select Print to PDF
3.Under Options-Select (View PDF After Printing)
4. It will ask Save Location Path
Try and let me know if you can able to send save are not.
Likely a bug / glitch.
Given that you cannot attach the workbook, I'd recommend you create a case with Tableau Support.