This is the 2783rd most frequent Spanish word.
mortal
"Subject to death" or "lethal."
The word 'mortal' here modifies 'error' to describe it as having fatal or very severe consequences.
Este error fue mortal para la empresa.
This mistake was fatal for the company.
In this sentence, 'mortal' is used as an adjective describing human beings as capable of dying.
Todos los seres humanos somos mortales.
All human beings are mortal.
Here, 'mortal' is used metaphorically to describe the challenge as extremely difficult, akin to being life-threatening.