I gifted an app but the recipient never received it

The recipient need to check the mail box, see -> Gift apps, music, movies, and more from Apple

Gift apps, music, movies, and more from Apple - Apple Support

• After the gift is sent, the recipient gets an email. When they open the email and click Redeem, the item automatically downloads to their device.

If you still need help, get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK)