This can happen if you are in a country that does not support the specific currency that you are attempting to use.
Here is a listing of Currency Codes
Also look at the Locale Code that is being used as well.
If you are using USD and the Locale Code or lc is US then you should be able to process USD with no issues.
<input type="hidden" name="lc" value="US">
<input type="hidden" name="currency_code" value="USD">
Thank you,
Jennifer
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