Themes must have at least one page container.
A page container is located within the body of your Splash page, holding all of its block content.
What is a page container?
A page container is within the body of your Splash page, holding all of your block content. If you do not have a page container, your page may experience serious technical issues.
In the main layout panel, click the square + button; this will open the block library. From there, click the Building Blocks category and then select Empty Page Block. This will add a new page container to your page. Once you've added this, simply drag all of your blocks into the new page container.