If you sided with the Dawnguard, while a Vampire Lord they will refuse any interactions with you, stating that you need to get cured immediately. She will immediately leave, even if she is currently following you, saying that she needs to do this alone. You should then be able to enter the Soul Cairn, but there's a catch. This starts Rising at Dawn. Once you're cured again, you can cure Serana. Another response appears if you've completed The House of Horrors :.
Zaniyah. Age: 23.
During the quest Chasing Echoes , she will offer to turn you into a Vampire Lord.
Blair. Age: 31.
Arrival at the Icewater Jetty :. Sneak , Conjuration , Light Armor , One-handed. She will no longer be undead, which means that she can be healed by your Restoration healing spells, she won't be damaged by Restoration combat spells that target undead, and she appears with Detect Life instead of Detect Dead. Outside of this, her necromancy is powerful enough to reanimate non-humanoid entities.