SharePoint Online Video Player URL update grey out under behavior tag

Hi all:

   I was trying to insert a video player web part into a SPO page, but after I did that, I found I cannot specify the URL and preview image URL for the video player, in other words, items under behavior tag cannot be updated at all. Does someone know how to solve this?

   And I'm trying to update the code of the web part in designer, but it tells me I cannot edit the code, which I have not met before, is it SPO configuration or something else?

   Thanks,

   Tony