Links to VZPP are missing in the POA Customer Control Panel on the VPS Management > General tab for a customer with VPS subscription.
The subscription does not have the 'Access to VZPP' resource.
Offline management is switched off for the VPS.
Find the subscription in question in the POA Provider Control Panel > switch to the 'Resources' tab > make sure that the limit for the 'Access to VZPP' resource is set to 1:
If you offer the access to VZPP for free, then consider to adjust the limit of the 'Access to VZPP' resource to 1 if it is set to 0, otherwise the customer has to purchase this resource through the PBA.
In the Provider CP go to Service Director > Virtuozzo Manager > VPSs > click on the VPS in question > General tab > click the Enable offline management button.