Do you have the Markarth Prologue quest CURRENTLY active, "The Ravenwatch Inquiry"? Or its sequel, "The Gray Council"?
If so, either drop or complete these quests: and then the Rivenspire quest "Dream-Walk into Darkness" should start properly.
This is a known and (badly) designed piece of quest incompatibility with Verandis Ravenwatch having to be present for one, but absent for the other: similar to the comparable incompatibility involving Abnur Tharn's rescue in the Main Quest (Harborage) and his appearance in the Elsweyr Prologue (The Demon Weapon / The Halls of Colossus), where if one quest is active, the other cannot be started.
Oddly enough, this was NOT done with either Sai Sahan (also Main Harborage Quest / Dragonhold prologue) or Lyris Titanborn (also Main Harborage quest / Greymoor prologue).
That;'s what the game *is* doing. Preventing you starting "Dream-Walk into Darkness" while "The Ravenwatch Inquiry" is active. If you had "Dream-Walk into Darkness" active first, you wouldn't have been able to start "The Ravenwatch Inquiry".