upload Screenshots keep failed on the App Store

Do you get error message ? What do they say ?

Is it only for iPad format or iPhone as well ?

Edited: you edited your question, so it seems it is also for iPhone format.

Edited2: apparently, you're not alone: https://developer.apple.com/forums/thread/727883

Which would mean there is a server side issue ?

It is evident from the web browser console that the issue is with Apple, but it is concerning that it has not been fixed for such a long time. The stability and performance are worrisome.

We believe we have resolved this issue. If you encounter issues with screenshots, you may need to delete and re-upload them.