"That is not ok, Fraser."
Translation:Chan eil sin ceart gu leòr, a Fhrìseil.
Frìseal is the nominative-accusative and dative form – used when talking about Fraser, when Fraser is the subject or the object of a sentence.
A Fhrìseil is the vocative – the form used to directly address Fraser, not talk about him.
You can access the tips and notes on the web browser version of Duolingo (unfortunately they are not available on the mobile Android and iOS app) and on the duome.eu website.
Other than the use of the particle and lenition, the vocative case isn’t what I would call “explained” in the tips for any module I have come across yet, and there is clearly more to it, eg Frìseal to Fhrìseil and others where an i is substituted or added in the final syllable. And then the issue of a before Fhrìseil because of the r in the first syllable, but not before Fheargais because of the ea! More complicated than the tips suggest and as an “older learner” I appreciate understanding the grammatical underpinnings.