In QinQ mixed VLAN mode

An interface (port or trunk) must be explicitly GVRP-disabled before it can be assigned to the S-VLAN space. When you first attempt to configure a port as tagged for an S-VLAN, the CLI will issue a message disallowing the configuration.

config# svlan 200 tagged a1,a2 GVRP enabled ports cannot be members of svlans. 
Disable the interface level gvrp configuration.

To disable GVRP at the interface, issue the following command:

config# interface a1,a2 unknown-vlans disable

When you configure the port, the CLI will issue a warning prompt:

config# svlan 200 tagged a1,a2 Ports a1, a2 will lose their cvlan memberships if any
Do you want to continue? [y/n]

Press Y to continue and automatically configure both ports as port-type provider-network (the default for all S-VLAN member ports).