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Step 3: Open the website and retry the checkout process again to verify the issue is fixed (picture 4)
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NOTE: If website only has 1 currency and you change base currency and allowed currency to another currency which is not USD. This issue still happens because of conflicting with Magestore Credit Card.
To solve it, there are 2 ways.
Access to all existing Gift code details and change the Currency field to configured base currency
2. Because it takes time to change currency field for all existing Gift codes => the 2nd solution is easier.
You need to go to Backend > Store > Configurations > General > Currency setup, always select USD as an option of Allowed Currencies besides the base currency. For example, if website only support AUSD (australia usd), you need to choose both USD and AUSD in the allowed currencies setup.
Then follow guide above to setup rate between USD and AUSD.