Make sure that your device is set to allow in-app purchases. Go to Settings → General → Restrictions and check that In-App Purchases is set to ON. Then quit and restart the game. Please perform the following steps.
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Double tap the home-button on your device.
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Swipe all application previews upwards until there is none left.
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Reboot your device.
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Launch Space Miner Wars and try again.
If you're still unable to buy Infinium, please contact our support team via the in-game Customer Support button located in the Options menu.