Seeing “Your security code could not be verified”
Normally when you’re sending end-to-end encrypted messages, your device automatically checks both your and your contact’s linked devices. Under rare circumstances automatic verification may not be possible, and you’ll see this security notification in your chat.
You can still continue to exchange messages with your contact. The verification process is optional, and is only used to confirm the messages and calls you send and receive are end-to-end encrypted.
If you want to manually verify your security code with a contact, you can tap the notification and follow the instructions on the screen. Learn more about verifying security codes in this article on end-to-end encryption.