You need a clear path to the target
The Grave Domain cleric allows you to cast spare the dying with a range of 30', instead of as a touch spell and as a bonus action if you want. The ability itself does not list any further requirements.
However there is a general requirement that you need a clear path to the target to target1 a creature (in chapter 10, "Spellcasting" of the PHB, p.204).
To target something, you must have a clear path to it, so it can’t be behind total cover.
A creature behind a corner would have total cover, so you cannot target it with the spell, and you cannot use spare the dying on it, wether you can see it, or not. (If the corner was of transparent crystal, it would still block an unobstructed path).
1 In case you wonder, spare the dying does not use the word "target", but a spell that affects only a single creature such as this would consider that creature its target (PHB p. 204)
A typical spell requires you to pick one or more targets
to be affected by the spell’s magic. A spell's description
tells you whether the spell targets creatures, objects, or
a point of origin for an area of effect (described below).
See also here for deeper discussion.