Currently, Odoo does not create an automated proration for the first invoice on a subscription without an Upsell.

We can use the following workaround to achieve our purpose:

When you create your recurring Sales Order, you can do one of the following:

1. Create a Service Product and add it to the Sales Order Lines.

2. In the Description you can specify the proration period and input the Unit Price.

3. This will carry to the Invoice when you create it and you can choose to invoice the Subscription and the Prorated Period product or only invoice the Prorated Product by itself first.

OR

1. If you don't wish to create a product and add it to the Sales Order Lines.

2. When you create the Invoice, Add a Line, and then in the Label add the description of the proration and the amount in the Price.

I hope this helps!