Is PostgreSQL Flexible Server is available in the Azure Native right now ? I have searched for it and couldn't able to find that. But got some information that it's available in a preview (@pulumi/azure-native/dbforpostgresql/v20200214preview). So is it ok the use that preview to create PostgreSQL Flexible Server ?

It's called Server in the API spec that we build from. Here's a couple links that include some related code: https://github.com/pulumi/pulumi-azure-native/issues/684 and https://github.com/pulumi/pulumi-azure-native/blob/master/examples/postgres/index.ts#L38-L52 and