Was this a testimonial outside Upwork or from a resulting contract or project on Upwork? If the testimonial resulted from a job on Upwork, it would take a few hours before it reflects on your profile. These days it takes more time, and I'm not sure why this happens.
To ensure trust and quality of our marketplace, we can only approve testimonials from clients where we have sufficient evidence that the contact information is valid. In this case, we may not have been able to find sufficient information. You can try requesting another from a different client. Thank you.