Jekyll 'language' versus Jekyll 'template'

I’m new to GitHub, and have set up a public repository on GitHub Pages aiming to link to it from IONOS’s “Deploy Now” service. The repository seems (to me) to be working fine, as far as it goes - just a few simple page stubs, written mainly in Markdown (a bit of HTML/CSS), made using Jekyll, with the ‘Just the Docs’ theme.

But when I try linking, it fails and IONOS tech support say “You need to convert your repository from the Jekyll ‘language’ to a Jekyll ‘template’ inside the Ruby language”. Can someone explain what that means, and what I’d need to do to achieve it?