Cases tab is an admin area only. To allow managers to access this tab, you need to escalate their access rights to this area:
To edit the access rights, follow the steps below:
Click the Access Button then click Employees.
Click the relevant employee record then click Edit Access Rights.
Select the relevant filters at the top to specify who this user will have access to.
Scroll down to Case Log then set its Accessibility: Update.
Click Save.