"It has a flat tire."
Translation:Elle a un pneu crevé.
The Swiss french Goodyear website has: https://www.goodyear.eu/fr_ch/consumer/learn/fixing-a-flat-tire.html#
Guide réparation de pneu plat
So why is
Elle a un pneu plat
But Duolingo teaches French regardless of region, right? So pneu plat should be accepted if that is used in a different French speaking region?
It may change in the future but in the past 6+ years, Duolingo has taught French from France (only).
Probably to make it an adverb meaning "on/down to/to... a flat surface".
It is also used figuratively: "je suis à plat" = I am shattered/bushed.
If "it" is "une voiture" (a car), you can use "elle" and if it is "un camion" (a truck), you can use "il". Both variants are accepted, as usual.
On many previous occaisions we've been told that if the gender is not obvious, the sentence defaults to masculine. Why is this "rule" not followed here, since there is no context so it could be either?
"It" is gender neutral in English but its translation can be masculine "il" or feminine "elle". There is no real reason to default to masculine since "une voiture" and "une moto" are feminine, while "un camion" and "un véhicule" are masculine. This is why both "il" and "elle" versions are accepted.