Its possible because of how emails are edited that an outgoing review request will not contain a valid review request URL. We recommend you reset your review_assignment email in this instance and here is a workaround to assist reviewers who have received the email without the link:
1. Go to the article review page
2. For the Reviewer having the issue select the View button
3. You'll see an Access Code like below
4. Click on this link and then copy the URL and send it to the reviewer - this will get them into the review.