Please allow an hour or two for the purchase to settle. If it still isn't working, try the purchase again.
If using a payment method that directs you to an external website (such as PayPal or WebMoney), you will be rerouted back to Steam after filling in your billing details. This will confirm your purchase. This process can take up to 60 seconds. To avoid purchasing failures, please do not hit your back button or close the payment window before this process is complete.
Additionally, you could try completing your purchase through a web browser instead of through the Steam application.
Source of the information and additional details can be found here.
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