"I had lunch with my lawyer yesterday."
Translation:Jag åt lunch med min advokat i går.
I noticed that this very old question hasn't been answered, so I'll give it a shot.
We don't use ha to mean 'eat' like you do in English. For eating, we say äta and for 'have' as in 'Have an apple!' we say ta (Ta ett äpple!).
luncha is a verb: Jag lunchade med min advokat i går is an accepted answer.
Footnote: It is possible to ha anything if you think of it as "a calendar event" and in that sense, you can certainly hear people say ha lunch – still, that's probably a less likely interpretation of this sentence and most importantly, could be misleading, so it's not an accepted answer.
Also an old question that needs a reply, I can see from reports that many people are thinking about this.
We do use the definite to indicate possession in some cases where English insists on having a possessive pronoun, but only in cases where it's clear from context. The perfect example is Jag borstar tänderna -> 'I'm brushing my teeth'. 1. Whose teeth do you expect me to brush? 2. Not everyone has teeth, but as a general rule, people are expected to have them.
If you're talking about a lawyer, none of those things apply. Why would I necessarily have lunch with my own lawyer? Not that obvious. And a lawyer isn't something everyone is expected to have. So because of this, if we said advokaten here, it would most likely mean 'the (previously mentioned) lawyer'.
Both i går and igår are totally correct. But the way this sentence is coded, the correct answer in multiple choice questions will only have i går. So the option with igår that you were shown must have had some tricky error. It's really hard to spot those sometimes, I'm not a big fan of the multiple choice questions myself.
There's also this bug concerning i dag in the course: https://www.duolingo.com/comment/12941839 – this is not what happened in your case, but could be useful to know.