Client access needs to be enabled for a Project in order for your Clients to view a specific Project, regardless of whether they have been added to the Project.
This is one of our security features that allows you to have more flexibility on dealing with customers viewing permissions.
Follow the steps below to check whether 'Client Access' is turned on for your Project.
Go to the Projects Page and select the Project that the Client needs access to
In the Projects Details Page, on the top right, click "Enable Client Access"