They had not had sin (hamartian ouk eichosan). Conclusion of condition of second class without an because context makes it clear (nun de) without it (Robertson, Grammar, p. 1013). The imperfect active indicative with -osan instead of -on (also in Joh 15:24) as common in the lxx, and occurs in the papyri and the inscriptions and the Boeotian dialect.
Excuse (prophasin). Old word (1Th 2:5) either from prophainō, to show forth, or prophēmi, to speak forth. Mere pretense, in John only here and Joh 15:24.