In Azure build pipeline i need to added the "Quality Gate Widget" task and its already installed, but i'm unable to see "Add" button.
Can anyone suggest me what needs to be done - is there any that i'm missin?
Unfortunately, that extension only contains a Dashboard Widget and not a pipeline task. So, you won't be able to add it to a pipeline.
You can use this Azure DevOps extension to fail builds in the event your Sonarqube quality gates don't pass: