You really have to read the GAO report itself.

The Department of Defense (DOD) is the only government agency that is incapable of being audited as the accounting system is, at heart, not really a system.

Here's the beginning of the GAO's comments: "The Journal Voucher Working Group (Working Group), which is comprised of Department of the Army (Army) and Defense Finance and Accounting Service personnel, since 2015 has been actively working toward implementing new processes to address inadequate support for journal vouchers (JV) in the Army’s general fund. JVs are accounting entries manually entered or system generated to record corrections or adjustments in an accounting system. From October 2016 to March 2017, the Working Group identified more than 121,000 unsupported JVs totaling $455 billion in one of its reporting systems, Defense Departmental Reporting System-Budgetary (DDRS-B)."

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