Would guppies spawn in a 2.5 gallon tank?
I would like to breed guppies to make some money (I am only 11 years old)
but I don't want my guppies to feel uncomfortable.

I wouldnt do it. Guppies are schooling fish and they need a minimal of 5 fish together to be happy. The 2.5 wouldnt be large enough for 5 fish, not to mention where you could raise the fry.

No, a 2.5 gallon tank isn't even suitable for guppies to live in let alone breed in. Guppies need 2-3 gallons per fish on average (1 gallon for every inch of fish). I'd say your tank isn't even large enough to house a single guppy and even if it were that would be a bad idea because guppies are social and like to have others of their kind around.

Breeding guppies isn't much of a money maker. Most petstores will only take them for free or a fraction of what they sell them for so maybe you'll be better off finding chores to do for your neighbors

I have my guys up for sale $1 each or $10 for a dozen, and people have asked about them, but no one's come to take them yet.