Here is some code that works for me on MacOS Monterey.

on closeNotifs(numTimes)
    repeat numTimes times
        try
            tell application "System Events" to tell application process "NotificationCenter"
                set boxes to groups of UI element 1 of scroll area 1 of window 1
                set numBoxes to count boxes
                repeat with i from numBoxes to 1 by -1
                    try
                        perform item 1 of (actions of item i of boxes whose name starts with "Name")
                    end try
                end repeat
            end tell
        end try
    end repeat
end closeNotifs

closeNotifs(2)

Instead of whose name starts with "Name"you may use whose name description is "Close" but I don't have English settings. I think this should work for any language.

I haven't solved the problem that the last notification is not closed automatically, so I basically run it twice.

This has been a PITA for so long.

Here is some code that works for me on MacOS Sonoma (14.2.1):

tell application "System Events" to tell application process "NotificationCenter"
    try
        log (actions of UI elements of UI element 1 of scroll area 1 of group 1 of window "Notification Center" of application process "NotificationCenter" of application "System Events")
        perform (actions of UI elements of UI element 1 of scroll area 1 of group 1 of window "Notification Center" of application process "NotificationCenter" of application "System Events" whose name starts with "Name:Close")
    end try
end tell

Instead of whose name starts with "Name:Close"you may use whose name description is "Name:Fermer" in French, or your own language. I added a log line so that you can check which action you wish to perform.

Anyway, this has been a PITA for so long, hope this helps.

Check this for the Monterey version: https://apple.stackexchange.com/a/467607/303345