Unfortunately, if you were hoping to collect the XP or other reward for completing the “Make a New Friend” challenge, you may have to wait some time before you can collect the reward.
Niantic has a series of known issues it’s addressing within the game, and this objective not marking as completed has been a common problem among players. You can contact Niantic support about it through the Pokémon GO app, but unfortunately, it seems to be a bug that the developers will have to fix.
Source: https://www.distractify.com/p/how-to-make-new-friend-pokemon-go