Hello Iris, I hope you are having a nice evening. Can you elaborate on what it is saying exactly as far as "cannot go through"?
How are you attempting to purchase the gems? Is this with a credit card or an iTunes card?
If your iTunes is connected to a credit card, it is possible the credit card is blocking the attempt as they believe the card may have been compromised. Often they will attempt to call you to confirm if it is really you making the purchase.
If I was helpful to you in this process, please rate me! If you have any other questions, please let me know and I will be happy to help. I would like to close out this ticket unless you have any other questions.