Multicurrency is available in the global version of QuickBooks Online, .
It's currently offered in both Essentials and Plus versions. It's possible that you're using Simple Start, which is why you're not seeing the option to activated Multicurrency.
You'll want to upgrade to either Essentials or Plus to turn it on. Here's how:
Once done, check out these articles as additional guides on how to use the feature:
I'll be around if you need anything else.