This is aircraft manufacture 101, and it happened on one of the first air frames that was supposed to been accepted by the Air Force.

Boeing really cannot make airplanes any more:

The delivery of a new KC-46 Pegasus tanker aircraft to Seymour Johnson Air Force Base in North Carolina has been delayed after foreign object debris was found inside the plane by Boeing inspectors.

The delayed aircraft was to have been one of the first two KC-46s delivered to Air Force Reserve Command at Seymour Johnson on June 12. But while the first was successfully delivered, debris was found in the fuel tank of the second during its pre-acceptance inspections.

In a statement provided to Air Force Times and Defense News on Monday, the Air Force said the debris came from “non-standard factory rework,” and not the kind of “production line quality escapes” that caused the Air Force to halt KC-46 deliveries in March 2019.

Those problems with foreign object debris, or FOD, led the Air Force to put a plan in place to correct the problem.

………

Just a month after Boeing began delivering the KC-46 to the Air Force in January 2019, the service found foreign object debris — tools or other materials used to build the aircraft — left behind in multiple KC-46s, creating a potential safety hazard. As a result, the Air Force stopped accepting new tankers over a weeks-long period in March and April 2019 as it investigated the issue.

I’m not sure that Boeing can be fixed without burning the whole rotten edifice down and rebuilding from the ashes.