Desktop version only

Hello, for 2 days I have been trying to make the site on mobile devices to be displayed only on the desktop version, no mobile version, but I can’t do it. I have another site where I have done it with this code

<meta name="viewport" content="width=device-width, initial-scale=1.0, minimum-scale=1.0, maximum-scale=1.0, user-scalable=0" />

but when I try to put the code in header.php it doesn’t work.