Thanks for your health query.
A gestational sac
greater than 21 mm, without visualization of a fetal pole
, is invariably indicative of a failed ( non viable ) pregnancy
Giving you the benefit of the doubt, your doctor has prescribed support for the pregnancy, in case there is a rare possibility of a delayed conception.
I would suggest waiting for maximum one week, and if the repeat scan too shows no fetal pole, a termination of pregnancy seems advisable.