When using the data command the game can't understand targeting tags for the entity.
Example: /data merge entity @e[type=zombie]
The feedback for the command is
"Only one entity is allowed, but the provided selector allows more than one
data merge entity @e[type=Zombie]<--[Here]"
Linked issues
Comments 3
However, you can place your cross hairs over an entity while typing the command and their target id will appear.
Intended. /data only supports modifying one entity at a time.
If you want to modify all zombies at once, you can use the following command:
/execute as @e[type=zombie] run data merge entity @s <nbt>If you want to modify one zombie, you can use the following command:
/data merge entity @e[type=zombie,limit=1] <nbt>
That is because @e[type=zombie] is referring to all zombies, not just one.