License key for single payment product

Unable to generate license key.

What I did:

1. Created a product (Single Payment, $6.99)

2. Created a "License Key" entitlement named "******** License Key" (2 activations, no expiry)

3. Attached the entitlement to the product

4. Went to License Keys → Create Manual Key → selected the product

Error received:

"License keys are not enabled for this product."

This workd in test mode but unable to do it on live mode. Please help!!!!