Adding an app on Modern Page - Sharepoint Online

[damansingh]

How to add an app on Modern Page - Sharepoint Online Page as for example, I created a Promoted Links App that I want to add to a Modern Page? Can I add it? 

[#2]

You will not be able to do that. Promoted Links is a classic interface web part. In order to add web parts on to modern pages, they need to be modern web parts (built with SharePoint Framework). 

In your case, you could use the Quick Links Web Parts for Modern pages.

[#3]

And you can add custom Apps or Add-in in modern pages too