What is the "What to do this week" box on my Marketing page?

It is a short list of quick wins, worked out from your store each time you open Marketing → Marketing Home. It shows up to three of these:

• Your catalogue is empty: Add a product.
• You have never created a coupon: Create a coupon.
• Your store has no default share image, so shared links post without a picture: Add a share image.

Each link opens in a new tab, so you keep your place on the Marketing page.

There is nothing to dismiss. Once you fix an item, it stops appearing, and when nothing needs fixing the whole box is hidden. That is why it disappeared after you added your share image.