E0148: A template file has Go and no markup

The markup of a .vuka.html file is what follows its <script lang="go"> (in the Go-header form, from the first line beginning with < outside the Go). The file has none: it is Go alone.

Wrong

<!-- page.vuka.html -->
<script lang="go">
package pages

type Page struct {
	Title string
}

func (p Page) Upper() string { return strings.ToUpper(p.Title) }
</script>

Right

Write the markup below the Go:

<!-- page.vuka.html -->
<script lang="go">
package pages

type Page struct {
	Title string
}
</script>

<h1>{Title}</h1>

or, when nothing renders, make it a .go file.