Final answer:
The missing punctuation is a comma after the word 'actions'.
Step-by-step explanation:
The missing punctuation in the sentence 'Her breathtakingly cruel callous actions led to her immediate resignation' is a comma after the word 'actions'.
The corrected sentence would be: 'Her breathtakingly cruel, callous actions led to her immediate resignation'.
The comma is needed to separate the two adjectives 'cruel' and 'callous' in order to clarify that both adjectives describe the noun 'actions'.