To access Facebook Marketplace you need to have an account on Facebook that is in good standing and not suspended.
You also need to be at least 18 years old and older and may need to verify your age as 18 or older before you can get access to the Facebook Marketplace.
You also usually need to have your Facebook Account for 4 months and have daily activity on the Facebook Account and have your phone number verified as well before the Facebook Marketplace will show up.
There are some people who have accounts for years on Facebook and they cannot access Facebook Marketplace but usually if you give it time and have an account it should show up for you.