A warm welcome to the Community, lspinola-skaibra. Let me help you sort this out.
Typically the error message you're seeing, Transactions can have only one foreign currency at a time, comes up when you're trying to enter a transaction using the home currency, a foreign currency, and one or more other foreign currencies.
To get this fix, you'll need to ensure you're creating one currency per transaction at a time.
It’s definitely worth taking a look at these write-ups to know more about how multi-currency works n QuickBooks Online. Also, please be reminded that once you turn on this feature, the home currency is set and irreversible:
Multi-currency FAQ.
Learn about exchange rates in QuickBooks Online.
You can always get back to me if you have any other concerns or have follow-up questions about QuickBooks. I'd be more than willing to lend you a hand.
