Can I choose "Artifact Creature" as a creature type as Adaptive Automaton enters the battlefield?
No, you can't, because "artifact" is not a creature type, it's a card type.

The information on the type line is divided into two portions, the type(s) and the subtype(s). Each word before the dash represents a type of the card, and each word after the dash (if any) represents a subtype. For example, Palladium Myr's type line says "Artifact Creature - Myr" so "artifact" and "creature" are its types and "Myr" is its subtype.

When a card says "creature type," it effectively means "subtype of a card which has 'Creature' or 'Tribal' as a type." In other words, the creature type is always a subtype, something that must appear after the dash.

Also, the complete list of possible creature types is given in the comprehensive rules, rule 204.3k as of this writing. When a card like Adaptive Automaton asks you to choose a creature type, you're only allowed to choose one of the words from that list (according to rule 204.3d).