Has your client checked their spam/junk folder to see if the email was sent there? If not, please send me a private message with your client's email address so I can look into this for you.
I have faced the same problem, then I tried the same thing you did, posting it in the community, but yet I didn't receive any solution or reason for testimony rejection, and tried to find my self,
I have helped the client with an application logo and branding, and he has an Upwork account as a freelancer, so l think that is the reason why they didn't accept the testimony, but I'm not sure about it yet as Upwork provided any exact reason.
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.
Currently, they'll need to have a LinkedIn profile.
Please note that your profile must represent you as an individual and that all the information in your profile including your name and profile photo needs to be accurate, truthful and verifiable. Please check this help article for more information on how to update your profile photo and this one for updating your name.
I have requested a testimonial from a previous client. However, they haven't shared it inspite of submitting the work. I now wish to delete that request from my profile. How do I do that?