I can not change currency on my site after the first currency's selection.
We have checked and found that the root of the issue is due to the currency rate is not updated after the extension is installed.
Could you please have a look on the screenshot below for reference?
For more information about product, please refer Checkout with Display Currency for Magento 2 page.
If you need any further assistance, please feel free to contact us.
Please visit our site for more useful Magento 2 Extensions: https://bsscommerce.com/magento-2-extensions.html