Transactions can have only one foreign currency at a time error. I already activated the Multi Currency in the account and settings

Good day, clients-financei.

Let me provide some information about adding multi-currency in QuickBooks Online.

You can only assign one currency to each account. If you wanted to use other currencies for transactions, you'll have to create a new account with the currency. Here's how:
 

1. Select Accounting, then hit Chart of Accounts.
2. Choose New.
3. Click the appropriate Account and Detail Types.
4. Provide the desired name.
5. Tap the Currency drop-down arrow, then choose the currency.
6. Click Save and close. 
    

I've also added this article for more information about Multi-currency: Multi-currency FAQ.

Don't hesitate to swing by anytime if you have other questions about multi-currency. I'll be around for you.