You may follow the below guide to add store credits to customer
Step 1: Navigate to Marketing > Store Credit & Refund by Aheadworks > Transactions

Step 2: Click Add New Transaction button

Step 3:  Set the value to add, set a note for reference then click Add Customer button
Step 4: Click the Filters tab, insert customer emails and click Apply Filters button to filter customer listing. Once the filter done, tick the checkbox on the customer record and click Add Selected Customers to apply.

Step 5: After you verified all the details is correct, click Save Transaction button to apply.
Once done, you can check back the transaction listing to verify the result or ask the customer check their account.