You need to add the provider alias to the databricks_metastore resource, i.e.:

resource "databricks_metastore" "this" {
provider = databricks.azure_account
name = var.metastore_name
storage_root = format("abfss://%s@%s.dfs.core.windows.net/",
azurerm_storage_container.unity_catalog.name,
azurerm_storage_account.unity_catalog.name)
force_destroy = true
owner = var.owner
}