Q:

Is it possible to manually trigger a stage when using YAML Azure Pipelines? 

With Azure DevOps release pipelines, it is possible to re-deploy a stage that either failed or succeeded.  With the new Azure Pipelines in YAML, I don't see any way to do that from the UI other than re-triggering the whole pipeline.  
Is that intended?  I would expect to see a "re-deploy" or "re-run" button on a stage like we had with release pipelines. 

A:

Although this is a basic function, I am afraid that unfortunately, at the moment it is not possible. 
There is a feature request about it, you can up vote there:

