Stefan is correct. See Exploiting AutoCorrect by Suzanne Barnhill, MVP.
You can use the AutoComplete feature with Building Blocks (including AutoText) so long as they are saved in:

1. a loaded Global Template
2. the Normal Template (this is the default storage location for AutoText)
3. the attached document template.

By the way. This can be very confusing because Word uses the term "Quick Parts" for the menu in the Insert tab. That menu includes access to two Building Blocks Galleriees (the Quick Parts Gallery, and the AutoText Gallery) as well as the Building Blocks Organizer. An image or text could be saved as an AutoText entry or a Quick Part entry in their respective galleries.
Where can Building Blocks be stored?

The difference between AutoCorrect and AutoComplete is that with AutoComplete you need to press Enter or the F3 key to get the entry. AutoCorrect replaces the trigger upon completion of typing the trigger and pressing space, Enter, or typing a punctuation mark. It does this without any notification, asking for confirmation or otherwise asking about it.