Answer:
The noun clause "that I had failed to set the alarm" is a subject complement.
Explanation:
A subject complement is a word, phrase, or clause that follows a linking verb and refers back to describe or complete the grammatical subject of the sentence.
In the given sentence, we have the linking verb is and the subject complement (the noun clause that I had failed to set the alarm) that describes or completes the subject of the sentence (The reason).
