Azure Synapse Workspace don't use managed virtual networks, this may make infrastructure security complex to manage.
The workspace network configuration cannot be changed after it is created; new resource must be created to enable the managed virtual network function. To do so, follow the steps below.
In Azure Console -
In Terraform -
References:
https://learn.microsoft.com/en-us/azure/synapse-analytics/security/synapse-workspace-managed-vnet
https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs/resources/synapse_workspace#managed_virtual_network_enabled