You need Dual PD or Monocular PD
Monocular PD is computed only with glasses wear on you
We recommend to find a way to have a eyeglasses frame, such link a family member one.
If you are wearing glasses but the result don't compute the Monocular PD, try with classical frame, try avoid rimless, or any "fancy" frames.
Monocular PD is a two-number value showing the distance from the bridge of your nose to your pupil on each side.
Dividing the single value PD by 2, should roughly, give you your Monocular PD for single vision glasses.
It could be considered as acceptable if your have small correction. We do not recommend to that in case of high correction or Progressive Lenses.
In this case please contact your optician to get his approval.