I couldn't choose any currency when creating a new customer. It's just an empty field in the form. How it could be fixed?

Hi there, 

QuickBooks makes it easy to manage customers using different currencies. I'll be happy to show you how that works. 

Based on what you've described, it sounds like you haven't turned on Multi-currency in your Settings. The multicurrency feature gives you the ability to record transactions in foreign currencies. If you have customers, suppliers, or bank accounts that use a currency other than your local currency, you can turn on the multicurrency feature to properly track these transactions, as well as exchange rate gains and losses. Once you turn the multicurrency feature on you cannot turn it off again, so be absolutely sure you want to turn it on.

The multicurrency feature and foreign currencies are available in QuickBooks Online Essentials and Plus. Once you turn multicurrency on, your Home currency is set and cannot be changed.

To turn on the feature, follow the steps below:

1.     Select Settings ⚙️.
2.     Select Account and Settings.
3.     Select Advanced.
4.     Select Edit ✎ in the Currency section and choose your Home Currency.
5.     Select the Multicurrency checkbox and confirm that you understand that you can't undo this option.
6.     Select Save.  

The Manage Currencies link that appears takes you to the Currencies page, where you can manage the currencies you use. For more info on Multi-currency, check out this article: Turn on and use multicurrency. 

Let me know if you have other questions. I'll be happy to help! :)