Can’t delete my chat archive

I’m the GM of a campaign and I rolled the initiative of some PNG and I don’t want that my players can read it. So I tried to delete the chat but even if I confirm that I want to delete, the chat can be read by everyone. What can I do to make my chat empty? Or to delete some messages?

If you are deleting the chat in game, you are just clearing what you see in chat (probably just for that session, but I haven't tested that).  Clearing the chat archive from the game launch page (in the settings drop down) will clear everything in chat history.  If you are logged into the game while clearing the chat archive in another tab/window, you may need to refresh the game to see the change.

I tried to do this by the main page but even if I confirm the chat archive doesn't be deleted (sorry for my english).