cannot add box to a layout through page composer

Summary:

cannot add box to a layout through page composer in a sandbox.

Content (please ensure you mask any confidential information):

We are trying to add a webpage onto the opportunity layout through page composer. When I tried that, it specified that a box needs to be added first with the following error message:

There are no boxes on the page, so you cannot add any content. Make sure your page (or the template it was based upon) includes at least one box before attempting to add content.

But when I try to add a box component, it shows the following error still. How can we add a box to start the configuration?