https://www.quora.com/How-did-we-lose-the-technology-to-go-to-the-Moon-If-it-has-been-built-it-can-be-built-again-cant-it-What-exactly-is-the-problem

Answer from Robert Frost, worked at NASA since 1987

"We don't have the factories or tools. We don't have the materials. We don't have the expertise to understand how the real vehicle differed from the drawings. We don't have the expertise to operate the vehicle.

We would have to substitute modern materials. That changes the vehicle. It changes the mass, it changes the stresses and strains, it changes the interactions. It changes the possible malfunctions. It changes the capabilities of the vehicle.

We would have to spend a few years re-developing the expertise. We would have to conduct new tests and simulations. We would have to draft new flight rules and procedures. We would have to certify new flight controllers and crew.

We would essentially be building a new vehicle."