Form not showing in the page

i’ve followed this guide to create a form in my website using the admin plugin in the page frontmatter:
but the page is empty, and the form is not showing up. I’ve also tried to copy the exact content of the example file ( via FTP but i had the same result, blank page.
I’ve also tried to make a fresh install of grav and add this page, but it still not work.
what is the correct procedure?

Thanks in advance

Did you install the Form plugin?

Yes is installed by default with grav admin