I get a notice that a main element is missing?

HTML elements define the structure of your pages. One of those tags is <main> defining the part of a page containing the content. Among others, this is relevant for accessibility, i.e. for skip links to skip menus or forms to jump to the right position. In Block Themes the main element often lies in your WordPress templates. When creating custom site templates, it may happen that no main element is defined. It can also happen that there are suddenly several main tags, e.g. after copy & pasting elements. But no worries. Greyd.Suite's got you covered and informs you if that is the case with a respective notice.

What do you need to do?

Open the respective template in the Site Editor. There, you usually have a header, content and footer element. The content needs to be in a group or similar. Click on that group and go to the advanced block settings on the right.

Here you can now select (or remove) the HTML element "<main">.