how to use Eraser in PowerPoint

As MS Office/PowerPoint is not an Apple product and there are not as many Office users active here as one would think, I believe you will be better served by asking in Microsoft's Office forums. MS Office questions posted in Apple’s forums can linger here a long time without getting effective answers.

Start here: 

https://answers.microsoft.com/en-us/msoffice

Be sure to include "mac" in your search string to limit the search to Outlook Mac topics. Same if you post a question—include "Mac" in the title. That makes it easier for other Mac users to search for new posts.

I believe you will find the contributors there knowledgeable and very helpful, much more so than MS's Windows-centric support documents.

Maybe someone here can assist you from personal experience, but …

As PowerPoint is a Microsoft product, consider contacting Microsoft Support staff or their support forums.