The noun phrase "a series of online tutorials" is the direct object of the sentence. The direct object is a word, phrase or clause that is the recipient of the action of a verb. A simple way to find the direct object is to ask what? to the verb. For example: Mara, a professional makeup artist, created what? - She created a series of online tutorials.
Only action verbs or transitive verbs can have direct objects. We must be careful not to confuse transitive verbs with linking verbs, since the last group would answer the question what? or who? with a subject complement.
