Niantic says the issue where the Referral Bonus does not work in Pokémon GO will be resolved in the upcoming version 0.331 update

Niantic is continuing to announce new events and content for Pokémon GO. Read on below to learn more:

Referral Bonus does not work

Last Updated: 2h

Issue description: When trying to add a friend using the Referral Bonus code, Trainers see an “error” message pop up and the referral bonus rewards are not added.
Issue status: Resolved in the next release (.331)

Source: Official Pokémon GO site

Source: Pokemon