Specifically for Xpax users, this might happen because of a few reasons:
1) The mobile internet plans that you want to purchase are not available anymore
2) Your SIM plan that you are using currently is not eligible for the internet plans that you want to purchase.
If that's the case, you might have to migrate your current SIM plan to a newer one If this issue persists, please submit a request and we will be right with you. Do note that non Xpax users will not be able to purchase any mobile internet plans.