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.
Double tap the home-button on your device.
Swipe all application previews upwards until there is none left.
Reboot your device.
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.