First, you have to enable sending license keys for the product/variation then create a generator rule. There are two ways to create a generator rule for a product/variation.

1. By creating a generator rule on the product page.
2. By creating a generator rule in the page License Manager > Add Generator Rule.

Product Page > License Manager section > License Generator tab:

License Manager > Add Generator Rule:

If “Active” is set to “No”, the license manager won’t generate any license keys.