Allow me to help and share some insights about viewing your clients message in QuickBooks Online.
Currently, being unable to view the link from activity notification email in QuickBooks Online has been reported as an ongoing issue. Our engineers are still working to get this resolved as soon as possible. For now, I'd suggest giving our Customer Care Team a call so you'll be added to the list of the affected users. Doing this helps you received an email notification regarding the updates of the issue.