I tried that.  It doesn't give me any option to manage any of them.  That tab just shows the subscriptions I personally have for the business such as loyalty and team plus etc

The hyperlink above will take you to online subscriptions @PoppyT, rather than Account & Settings > Business > Pricining & Subscriptions. You can also get there from the menu:

While it's not possible to create a 100% discount for a subscription payment, you could still offer a discount on an upcoming subscription payment. The minimum amount to be paid on a subscription / invoice would be $1. To get started: 

1. Select a current subscription. 
2. Click on an upcoming or outstanding subscription payment.
3. This will take you to Square Invoices. From here, select > Edit.
4. Select > Add discount and create a discount.
5. Once you're done, select Update. You'll be able to send your customer an update from here, too. 

Let us know if this could be a workable workaround for you!