Not having full ankle rom will impact your ability to to do a pistol.
If you have a stable bottom position of a pistol that is a start.
If you are not able to create a strong stable bottom position you will not be able to perform a pistol.
If you are missing ankle rom mobilize at the position of restriction.
Another impacting factor could be your hip mobility.
Squat Quick Test: Is it your Ankles or Hips?
Episode 320: Epic Pistol 10 Min Squat Test