How can I save an estimate without errors? every time receive the: "Something's not quite right Transactions can have only one foreign currency at a time."?

Hello there, @lumidense.
 

In QuickBooks Online, to save the estimate you created. You’ll want to use the same currency.
 

You can only assign one currency to each account or contact name, and you have to add a new account for each different currency that you will use in transactions.
 

In the account you created for your customer, please ensure that you set up the currency the same.

I attached a screenshot below for your visual reference:

Let me share this article for additional information About Multicurrency.
 

If you have other concerns, we'll be around to help you some more. Thanks.