Yes, you can sell monthly subscriptions on Shopify. You would need to use a Shopify Payments account and configure your shop to offer recurring payments. This can be done using the Shopify Billing app.
To get started, you first need to create a product in your Shopify admin. This product will be the subscription that your customers will be able to purchase. You can either create a new product, or use an existing one.
Once you have created your product, you need to enable Shopify Payments for your shop. To do this, go to the Shopify Payments settings page in your admin and follow the instructions.
Once you have enabled Shopify Payments, you can configure your shop to offer recurring payments. To do this, go to the Billing settings page in your admin and select the ‘Recurring charges’ option.
Now, when customers purchase your subscription product, they will be charged on a recurring basis according to the schedule that you have configured.