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Hello

I am facing a problem with coupons on my 3.0.3.2 installation.
When I enter a coupon code either in the cart or during checkout above my payment methods I get the following errors at checkout (checkout even doesnt get through):

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Payment Method
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Please select the preferred payment method to use on this order.

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I found out it is related to the taxes. It only happens when I have select the customer group in settings --> Localisation --> Taxes --> Tax Rates under my only Tax Rate.
If I have set the tax rate off this error doesnt occur.

Any idea how to solve that?

Have you removed any tax class recently? It appears your products are assigned to the tax class that does not exist.

Do you have any coupon extensions installed?
And what other extensions do you have?

At the moment it looks like the problem comes from a specific payment method module. As soon as I disable that module the problem does not exist anymore.

good job for finding that ..
if we are to assume this is an extension and not opencart ..

Contact the developer and ask them to fix ..