Virtual IP address option does not appear when setting up Solarwinds High Availability

The requirements to setup and configure HA in cloud and also answers the question why the Virtual IP Address Option is not available when Setting up an HA pool.

This issue can occur when deploying SolarWinds High Availability in different subnets. The virtual IP address option will only appear if both pollers are in the same subnet.

According to: When do I use a VIP or a virtual hostname? doc page

A Virtual IP (VIP) address is an IP address that is shared by both members of a HA server pool on the same subnet. When a member of the pool goes down, the other pool member takes over the VIP address and responds to requests sent to the VIP. The VIP and each pool member must be part of the same subnet.

• Starting in SolarWinds Platform 2025.1 and above, SolarWinds High Availability  supports IPv6 addresses.
• The VIP option is not available when running HA in the cloud. Instead, you must use a virtual hostname.

And according to: Task 10: Enable High Availability (optional) of Deploy SolarWinds Orion Platform products to Microsoft Azure doc page
Azure does not support the use of a virtual IP address. As a result, Orion High Availability cannot be deployed in Azure if both members of the same HA pool also reside in the same subnet. Each member of an HA pool must be deployed into a different subnet. A virtual hostname (using, for example, Azure DNS) can be used to direct users to the active member of the HA pool. See Azure's help for more information.

That means you have to use the Virtual hostname and must use the MS DNS or Other DNS option during HA Pool Configuration. See below article for more info
Required DNS Permissions to set up a High Availability Pool and access Microsoft DNS (solarwinds.com)