KJV: But he denied, saying, I know not, neither understand I what thou sayest. And he went out into the porch; and the cock crew.
NIV: But he denied it. “I don’t know or understand what you’re talking about,” he said, and went out into the entryway.
ESV: But he denied it, saying, “I neither know nor understand what you mean.” And he went out into the gateway and the rooster crowed.
CSB: But he denied it: “I don’t know or understand what you’re talking about.” Then he went out to the entryway, and a rooster crowed.
NASB: But he denied it, saying, “I neither know nor understand what you are talking about.” And he went out onto the porch* — with the footnote, Later mss add and a rooster crowed).
The manuscript evidence is split between including and omitting the phrase. Aland slightly favors the inclusion of the phrase.
A decision here is difficult. In the parallel passages we read about the rooster crowing (Matthew 26:74 and Luke 22:60), and in the following verse in Mark we read that the rooster crowed twice (Mark 14:72). The suggestion is that the phrase was added in v 68 to describe the first time the rooster crowed.
UBS gives the inclusion of the words a C rating.