/u/solipsist, in another comment on this thread:

Do not try to obtain Sstone yoursself. I forbid.

This was said by Quirrell in Parseltongue. If you can only tell the truth in Parseltongue, then Quirrell was really forbidding Harry from obtaining the stone himself.

If Quirrell can't lie in Parseltongue (and not just Harry, since Harry's speaking as a standard Parselmouth but Quirrell is speaking as a sentient snake), and if that prohibition enforces the sincerity of imperative commands and not just declarative statements, then clearly what Quirrell is saying is that Harry should try to make his own Philosopher's Stone.

"It's not a secret." Hermione flipped the page, showing Harry the diagrams. "The instructions are right on the next page. It's just so difficult that only Nicholas Flamel's done it."

"Well, it can't work," Hermione said. She'd flown across the library to look up the only book on alchemy that wasn't in the Restricted Section. And then - she remembered the crushing letdown, all the sudden hope dissipating like mist. "Because all alchemical circles have to be drawn 'to the fineness of a child's hair', it isn't any finer for some alchemies than others. And wizards have Omnioculars, and I haven't heard of any spells where you use Omnioculars to magnify things and do them exactly.

So the first thing Hermione mentions as a limitation of doing alchemy is the insane precision of the circle you have to draw. But what if there were already an acceptable, permanent alchemy setup just lying around somewhere where Harry could get to it?

The three of them stood within the Headmaster's private Transfiguration workroom, where the shining phoenix of Dumbledore's Patronus had told her to bring Harry, moments after her own Patronus had reached him. Light shone down through the skylights and illuminated the great seven-pointed alchemical diagram drawn in the center of the circular room, showing it to be a little dusty, which saddened Minerva. Transfiguration research was one of Dumbledore's great enjoyments, and she'd known how pressed for time he'd been lately, but not that he was this pressed.