‘Artifact’ is a type, not a subtype. When an effect has you choose a subtype, like a creature subtype, you cannot choose artifact. This also applies to effects that depend on or apply to specific creature subtypes.
Years ago, it was more common for an artifact creature to exist without a creature type. However that is no longer true, and all Artifact Creature cards currently in standard have printed creature subtype. It is possible for Coat of Arms to increase the power and toughness of Artifact Creatures, but it will specifically focus on the subtypes of those Artifact Creatures. For example, if just a Coat of Arms, a Snapsail Glider and Hexplate Golem are in play, then these two creatures will not receive a bonus to their power and toughness. This is because the Glider is a Construct and the Hexplate Golem has the the creature subtype of Golem. If instead the Hexplate Golem were a Spin Engine, then the Glider and Spin Engine would both receive +1/+1 from Coat of Arms’ ability, as both are Constructs.