This is another finding from deploying Dynamics 365 PSA. So, during the testing phase, I noticed the test user account could not see WBS link under project entity.
The copied security role assigned to the test user has complete (Create, Read, Write, Delete, Append, Append To, Share and Assign) to Project and Project Task entity. The security role was copied from Sales Person as the role is more specific for sales scenario.
Upon investigation, apparently for this role to be able to see the WBS is to add ISV Extensions privilege