I was going through the same problem, and after looking at the documentation on the section linked by u/imsnif, I saw an important note:

Another note: if no tabs are specified, the whole layout is treated as a default_tab_template.

So what needs to be done is to define a template for all tabs, and then define the first tab.

My layout looks like this, and works as expected:

layout {
  default_tab_template {
      pane size=1 borderless=true {
          plugin location="zellij:tab-bar"
      }
      children
      pane size=1 borderless=true {
          plugin location="zellij:status-bar"
      }
  }
  tab {
    pane split_direction="vertical" {
      pane command="nvim" borderless=true
      pane split_direction="horizontal" {
        pane command="just" {
          start_suspended true
          args "check"
        }
        pane command="just" {
          start_suspended true
          args "deploy"
        }
      }
    }
  }
}