The enchantment ID for Protection is minecraft:protection. Java Edition 1.13+ commands such as /enchant use this namespaced form. On Java 1.12.2 and earlier the numerical ID was 0.
What is the max level of Protection?
Protection goes up to level IV (level 4). Run /enchant @p minecraft:protection 4 while holding a valid item to apply the maximum level.
What does Protection do in Minecraft?
Protection is an enchantment in the armor group, applying to armor pieces. It can be applied up to level IV.