the Google pageSpeed reported that my site needs viewport, that it is not set
How can i do it?

I found page with details https://developers.google.com/speed/docs/insights/ConfigureViewport

They mention html tag:
<meta name=viewport content="width=device-width, initial-scale=1">

I added this tag to my site header, but google pagespeed still complaining..

Here is a nice tool to see how website will look like on mobile devices: