how can I delete all photos on I cloud

Go to settings > your name > iCloud > Photos >turn off iCloud photos. This will prevent future photos from saving to iCloud.

To delete the photos saved to your latest iCloud backup go to settings > your name > iCloud > manage storage > backups > select the latest backup for your device > switch off photo library. I believe this removes photos from your last iCloud backup but remember removing photos from the iCloud means that if you ever have to restore your device, your photos won’t be there unless you transfer them to a Mac or a laptop.

If you have a computer, open Photos, go to Photos/Preferences/iCloud and set up syncing. Then go to System Preferences/iCloud, sign in, and check Photos syncing. Once all the Photos have synced to your computer, quit Photos. You should now have a complete compilation of your photos (like a Master Library). Reopen Photos with the option/alt key held down and create a new system Library. You can now delete photos from your devices while maintaining a master copy on the computer. Added photos can be exported from the new Library and imported into the Master Library. Turn off iCloud syncing when reverting to the Master Library.

Photos - Use multiple libraries in Photos

To delete from iCloud.com/Photos, make sure device/computer syncing is off. The only way I am aware of is to do a manual delete at iCloud.com/Photos. Hold down the command while selecting the photos. Then click the Trash Can. You can also try selecting a moment, then click the Share button, and then cancelling the Share. The photos should remain selected.