You can only send a request to buddies who are active in the same categories as you.
For example:
If your habit is in Health & Fitness, the Buddy must also be in Health & Fitness.
This makes sure that you request buddies who are focused on the same areas as you.
If you try to send a request to a Buddy in a different category than yours, you will see a tooltip that says Activate category to request.
To send a Buddy Request to that person, you either need to
change your active category or
earn 400 points in your active category to add another category.