I am sorry to hear that you are closing your account. If you have any feedback for us or would like any assistance that would make your time at Shopify better, please do let me know!
It sounds as though you have an outstanding invoice that is attached to your account. Therein, your March invoice has been unpaid and so you are unable to close your account. You can check out your billing information from Settings > Billing. Your most recent invoice should appear there. For help in better understanding your bill please read this guide.
There are two options for you here:
1). If you are trying to deactivate your Shopify account and have an overdue bill, you will need to pay the bill first to regain access to your account in order to close it down. You can pay the outstanding amount and then close your store as per this guide. Once your store is closed you can contact support through the Help Center. Shopify Support will then look into getting you a refund on the subscription amount that you paid.
2). You can leave the amount unpaid and your account will be frozen after our last billing attempt. If you missed a payment or if a payment fails three times, then your store will be frozen until you settle your bill with Shopify. Your store does not freeze until your bill’s due date. If your store has been frozen we will no longer try and charge you, you will no longer have access to the admin and your storefront will not be shown any longer. It is effectively closed. If at any point you want to unfreeze your account you can do so by logging in and paying the outstanding balance.
Let me know how this goes and if there is anything else I can assist you with, I’m happy to help!