The description of the spell Revivify does not require that the deal creature's soul be 'untrapped'. While other resurrection-type spells do mention that requirement, those descriptions only apply to those spells.
Revivify itself only requires that the creature have died from some cause other than old age, and that the death have occurred less than one minute before.
When a creature dies, its soul departs its body, leaves the Material Plane, travels through the Astral Plane, and goes to abide on the plane where the creature's deity resides. If the creature didn't worship a deity, its soul departs to the plane corresponding to its alignment. Bringing someone back from the dead means retrieving the soul from that plane and returning it to its body.
This description of Bringing Back the Dead from the DMG has been mentioned to suggest that trapped souls can not be revived, because they do not meet the requirements here. There are several problems with taking the criteria here as literal rules, rather than general background, however.
The first problem is that it requires that the creature's soul be on either the plane of the creature's deity or its alignment plane. If the soul was anywhere else, even if not trapped, then this 'rule' would require that all Resurrection-type spells fail.
Second, in a similar manner, any spell effect that blocked planar travel would also cause all Resurrection-type spells to fail, since the soul would be unable to be "retrieved" from the planes as this 'rule' would require.
It seems odd to have spells like Private Sanctum or Hallow prevent spells like True Resurrection from working.
There is also the fact that the other spells do, explicitly, lay out the untrapped requirement. At the same time, spells and effects that trap souls usually state, explicitly, that the creature cannot be restored to life while the effect is in place (Soul Cage, early edition Demi-Liches, etc). The presence of that effect in each of the spell/effect descriptions strongly suggests that there was no general rule for the writes to fall back on.
It seems clear, from the wording of other Resurrection-type spells and soul-imprisoning effects, that souls considered "trapped" are not eligible for returning to live. It seems an accidental oversight and for balance purposes it seems best to treat Revivify as if it has the same "no trapped souls" restrictions as other Resurrection-type spells.
Attempting to twist other rules into covering for the flaws in this one spell description would have worse side effects.