The `less` utility will try to open the file(s) listed as operands on the command line. There is no file called `/header` on your system. What you tried to do what to give the interactive command for searching for the string `header`, but this can't be done that way from the command line.

Any interactive `less` command can be given as an initial command for `less` to execute by prefixing it with `+` on the command line. So you could have done
```sh
wget --help | less '+/header'
```


See `man less | less '+/ \+ '` for more info about that.


This happens to be equivalent to the other way of specifying a search pattern on the command line, `-p pattern`, but is more generic as adding an initial `+` applies to all interactive commands, while `-p` is specifically for specifying a search term.

```sh
wget --help | less -p 'header'
```