Ich möchte bitte zwei Fahrkarten kaufen.

[details=“English Translation”]I would like to buy two tickets please.
[/details]

But two tickets for a concert would not be Fahrkarten and there was an offer of Karten which I selected and it was considered incorrect…

Fahrkarten for a ride in the bus makes sense. But you’re right. Ich hab Karten für’s Konzert - I have tickets for the concert - is legit.