In order to assign Store Credits to the user’s account you have to create Store Credit Categories. The category defines the reason the user received Store Credits to spend in your store.
To find Store Credit Categories, click on the Configuration dropdown and look for the Store Credit Categories option.
There’s now a simple way to create a new Store Credit Category. You can do it by pressing the New Store Credit Category button in the upper right corner.
There is only one input box that tells what kind of category you will create. Simply input the name and press the Create button.
The previously created Category is visible in the Store Credit Categories settings. You can now create more of them or make additional changes to existing ones. In every other section in Spree you can see Edit and Delete buttons next to the name of category.
You can now assign Store Credit to the User’s accounts. You can learn more about it here