enable stp
config stp version mstp
config stp maxage 20 maxhops 20 forwarddelay 15 txholdcount 6 fbpdu enable     
config stp priority 32768 instance_id 0
config stp mst_config_id name technor revision_level 5
config stp ports 1-25,27 externalCost auto hellotime 2 edge true p2p auto state disable
config stp ports 1-28 hellotime 2
config stp mst_ports 1-28 instance_id 0 internalCost auto priority 128
config stp ports 1-25,27 fbpdu disable
config stp ports 1-25,27 restricted_role true
config stp ports 1-25,27-28 restricted_tcn true
config stp ports 26,28 externalCost auto hellotime 2 edge auto p2p auto state enable
config stp ports 26,28 fbpdu enable
config stp ports 26,28 restricted_role false
config stp ports 26 restricted_tcn false

config qinq ports 1-7,9-28 role nni outer_tpid 0x8100 trust_cvid disable vlan_translation disable
config qinq ports 8 role uni outer_tpid 0x8100 trust_cvid disable vlan_translation disable

DES-3200-28:4#enable qinq
Command: enable qinq

You should diable stp at first!

Fail!