Could you please share the screenshot for the same. Normally in Tableau Desktop it wont ask to save to online, from Desktop we only can publish to Tableau Server or Tableau Public.
May be it is a Tableau Public Desktop. Try to download the Tableau Desktop (current version) from below link:
Tableau Desktop | Tableau Software and activate your License.
You could save to public, download it as a TWBX, delete from public and then open in desktop?
Thanks Tom. That worked.
I wasn't aware that twbx files were downloadable from public.
You saved the day.