She sued the company for failure to repay debts of more than US$50000.

划线部分作 failure 的定语。

for failure 是 sued （控告）的原因。“因未能偿还5万多美元的债务” 而控告公司。

failure 的动词是 fail。 fail to do sth. 未能做成某事。虽然 to repay debts of more than US$50000 是 failure 的定语，但 failure 与不定式存在“动宾关系”。所以，译为“未能偿还...”