Hello,
I saw the screenshot you attached. This error message is displayed when there is no content added to the page.
Basically, The Shop page is an archive page and its controlled by woocommerce, it won’t save content that you manually add to it thats why its behaving like this.
Please elaborate with the help of some relevant screenshots, which changes you want to save to the shop page to check it myself and help you out accordingly.
Regard’s.
Xtemos Studio.