By the mid-1920s, German money was worthless chiefly because of
Inflation
Universal healthcare
Counterfeiting
Corruption

Inflation

Explanation:

During the early 1920’s the Weimar Republic (German government from 1918 to 1933) was affected by Hyperinflation*, particularly in 1923. This happened because Germany had many debts they could not afford: a) the Reichstag (German parliament until 1918) funded the costs of WWI by borrowing money, which they could not repay after the War, as Germany was defeated by the Allies and could not annex the rich territories they tried to occupy; b) after the Great War the debt was increased as the Allies imposed very large reparation sums to be paid by Germany (Treaty of Versailles and London Payment Plan).

With the London Payment Plan, Germany had to repay the money in gold or foreign currency in annual installments. When they started the repayments in gold marks, during the summer of 1921, the paper mark started to lose value because after the repayment they started to buy foreign currency at any rate, which started to depreciate the paper mark. This caused that by 1922 Germany was not able to buy foreign currency or gold in paper marks, so they had to start exchanging them for goods; and so, they were not able to make the repayments.  Then, in 1923, to ensure Germany paid the reparations agreed France and Belgium occupied the Ruhr valley, which prompted workers to go on a strike. This meant that there was no income from production. So Germany had to print more paper marks to pay for salaries, which inundated the market with paper marks, devaluating the currency and creating a hyperinflation. By November 1923 a USD was equivalent to 4,210,500,000,000 marks.

* Hyperinflation: when inflation is very high and happens in a very short time. As the general price of goods and services increases, the real value of the currency highly decreases. The purchasing power of the currency decreases. This means that, for example, with one dollar you can buy less things than before inflation. Therefore, people cannot buy essentials as their prices become exorbitant.