How i can remove search box in header?

Unless you provide a link to a page demonstrating the problem, no one will be able to offer much in the way of assistance.

https://www.cpaofferproficient.com

Here you go!

Do not edit the Twenty Eleven theme. It is the default WordPress theme and having access to an unedited version of the theme is vital when dealing with a range of site issues. First create a child theme for your changes.

Paste this code `#branding #searchform {
display: none;
position: absolute;
right: 7.6%;
text-align: right;
top: 3.8em;
}`

inside style.css of the child theme.