The cause is that the VLAN used by if-match vlan-id is different from the the VLAN to which the traffic policy is applied. The traffic policy can be successfully applied in a VLAN only when the VLAN used by the traffic classifier is the same as the VLAN to which the traffic policy is applied.

[HUAWEI]traffic classifier c1

[HUAWEI-classifier-c1]if-match vlan-id 10
[HUAWEI-vlan10]traffic-policy p1 inbound

The traffic classifier c1 defines a matching rule based on the VLAN ID.

[HUAWEI]traffic classifier c1

[HUAWEI-classifier-c1]if-match vlan-id 412 to 413

When the traffic policy associated with the traffic classifier c1 is applied to VLAN 412, the system displays an error message:
[HUAWEI-vlan412]traffic-policy p1 inbound
Info: Operation maybe time-consuming, wait...
Error: vlan conflicted, on slot 0 vlan 412.