Can I write and send letter from my iPad Pro

What do you mean by “write”, exactly?

You can type out emails in any email app, even Apple's own Mail app.

You can use any other type of text typing app,,like the free Apple Notes app or any type of word processing or notes style type apps.

All found in the Apple iOS/iPadOS App Store app.

If you are inquiring about actual handwritten letters on an iPad, using an Apple Pencil, there maybe seperate apps to do the actual handwritten letters/notes, but the result may have some extra steps to be able to send a handwtitten letter or notes in an email app/service.

Again, such apps can be found in the Apple App Store app.

Best of Luck to You!

You can write letters in any app, I use apple notes but there are many good writing apps available in the App Store