The vSwitch drop-down list is empty or missing a vSwitch you already created when you try to create a Tair (Redis OSS-compatible) instance, change the network type of an instance, or migrate an instance to another zone.
Zone or VPC mismatch (most common)
The vSwitch exists in a different zone or VPC than the one you selected on the instance creation page. Change the VPC or zone to match where your vSwitch resides, and it will appear in the list.
vSwitch belongs to a different account
If the vSwitch was created under a different account, it will not appear regardless of the zone or VPC you select. Create a new vSwitch in the correct zone under your current account. For instructions, see Create and manage a vSwitch.