If you're getting "payment unsuccessful insufficient funds" as a payment error, then you would need to ensure you have enough in the bank account to cover the converted amount. Your credit card may charge an additional fee for the conversion, which could also cause an issue. Your best bet here would be to contact your bank as the error you're seeing is the only information the bank provides to Shopify.
If you're using a debit card with credit card capabilities (i.e. Visa Debit), then this issue most likely because the card issuer has disabled one of the following transaction types:
If the card does not work, then you would want to look into using a different credit card or using Paypal (if it is eligible in your region).
Let me know if you have any questions.