Same problem here as well. Changing the "Base country or region" to the same currency as the phones appstore resolves the issue. But that is still troublesome since our company has testers from multiple countries.

For developers who are having this issue with their Testflight builds i've found a workaround. Go to the In-app page. Select you In-app Press the "Price Schedule" + button Select the "Custom Price Change" Set "Effective Date" to "Make Price Change Now" Choose "Manually manage prices in specific countries or regions" Select the countries where you have Testflight users. Choose a price.

Getting the same error as well!