In order to send a collab request to someone, the Model you are looking for needs to have the blue verified check on their profile and be opted in to receive collaboration requests.
If the Model you are looking for just opted in, it will take approximately 1 hour before they can be searched with the "Send New Collab Request" Button
The Model you’re searching for might not have opted-in to be a Collaborator yet. To become eligible to receive collaboration requests, they first need to opt in.
If they haven’t done so, you’ll receive an error message indicating that tagging is disabled for them.