Welcome to the Community.
This feature is only available in Essentials and Plus versions. For Simple Start users, you have the option to select your Home Currency. But, won't be able to set up multi-currency.
You may upgrade so you can use it. Just go to the Billing & Subscriptions section and click on the upgrade link. Here are some helpful articles about this feature.
Multi-currency tips and tricks
Let us know if you have any questions. We'll be right here to help.