Really intelligent and educated English speakers still make this very basic mistake.
I'm sure that you don't but let's review the rule.
As I'm SURE you know, the apostrophe replaces a letter:
Now, I know it's confusing when some possessives also need an apostrophe:
But in order to avoid confusing this possessive 's with the 'is' apostrophe, someone must have decided that pronoun possessives do NOT need an apostrophe!
These days, many people make the 'it's/its' mistake and you might think it's not important.
But if it drives me crazy, I'm sure most IELTS examiners think the same!