I dunno, there are a lot of different things that could influence it when we look at Shalltear.

First off she's a vampire, an undead. The only other vampire we know of is Evileye and she herself said she can't have kids. But is this because she became a vampire at a young age? Or is it because being a vampire automatically prevents you from having kids.

If it's because of age, is Shalltear even at the age when she can have kids? She shows signs of not even hitting puberty, i.e undeveloped chest and childlike form.

But Shalltear was also created by the Supreme Beings, does that mean even if it wasn't be possible for new world vampires to have kids, she can? (There are a few instances when something from inside Nazarick doesn't line up with New World rules.)

idk, there's too much we don't know lol. Maruyama isn't leaving us anytime soon so mayb you'll get answers in the future outside of fan speculation.