The failures of the Weimar Republic were due both to internal and to external factors.
The external causes are well-known. They stem from WWI and the Treaty of Versailles. The treaty forced Germany to pay reparations that seriously harmed the country’s economy. The Great Depression also helped to ruin the economy. Neither of these was an internal factor.
But there were internal factors as well. The constitution had seriously problems. In particular, it gave the president too much power. It allowed him to appoint and remove the chancellor. It also gave him the right to rule by decree in the case of an emergency. Thus, there were also internal problems that contributed to the failure of the republic and the rise of Nazism.