The short answer
Yes, contractions are permitted in CELPIP Writing. Contractions like 'don't', 'it's', and 'I'm' are common in professional English and won't reduce your score.
When contractions help
Contractions make your writing sound natural and flow better, especially in Task 1 (email). Examiners expect natural English, not overly formal academic writing.
One caution
Using too many contractions can make Task 2 (survey essay) feel casual when the prompt requires formal opinion. Balance is key: use them naturally based on the task's tone.