It is achievable with existing technology and materials. We would need something like an industrial lunar base to get material cheap into orbit, though.

A reusable Starship being churned out one a day at the Starfactory can achieve that:

A Kalpana One space habitat is estimated to require a mass of 10 billion kilograms, or 10 million tons.

One Starship launch can place 100 tons in orbit. 10,000 Starships, launching once a day, can place 10 million tons into orbit in 10 days.

From there, tapping extraterrestrial resources becomes far more feasible, and using them to construct an O'Neill Cylinder comes within sight.

Sources:

Peter Hague's Planetocracy blog provide the Kalpana One mass estimates: https://planetocracy.org/

Starfactory planning to manufacture one Starship per day: https://www.space.com/spacex-starship-one-a-day-starfactory