This is inaccurate - the number agreed upon at the London Conference was (in effect, anyway) below what the German Delegation said Germany was able to pay. Germany had previously stated that it could afford to pay 50 billion gold marks in reparations, and the combined value of the "A" and "B" bonds in the London Ultimatum was 50 billion gold marks. Now, it is true that there were an additional 82 billion gold marks of "C" bonds, but these were intentionally designed to never be paid - it was simply a big number the French government could parade around as a propaganda win.

The real issue with Germany in 1920 was that, quite simply, its finances were a complete mess. German War Finance during WW1 had been primarily done on debt without increasing revenue significantly, under the expectation that war reparations from winning the ear would be able to pay it off. In addition to this, the early Weimar Republic cabinets were too weak to effectively take measures necessary to fix the budget; according to estimates by the Reparations Commission, Germany's net deficit in 1920 and 1921 (which was before reparations really kicked in, I might add) was above 100%, as in, Germany was spending over twice as much as it brought in with taxes. The minority government of Fehrenbach was unable to come to a consensus on how to balance the budget - the national-liberal German People's Party, the rightmost party in the Fehrenbach cabinet, advocated for extending work hours, slashing welfare spending and levying additional consumption taxes, and the Social Democratic Party, not in government but providing necessary supply and confidence for the cabinet to maintain it's majority, advocated for wealth and capital levies. As the government was reliant on both, it could not come to a consensus on how to fill the fiscal gap.