hide category header

.category.archive header.page-header {
    display: none;
}

To add or override CSS: use the “Additional CSS” option in the customizer. https://www.ads-software.com/support/article/appearance-customize-screen/

Learn to use the Chrome Developer Tools or Firefox Developer Tools to help you see and test changes to your CSS.

thank you i understand you now
but which file is that please ?

Use the customizer, as I linked.

im trying to insert the css code you gae me by inspector but nothing work

and the customize in oceanwp not allow me to save the css code

Please post a screen shot of that. (Use imgur.com for the image.)

Or, I recommend asking at https://www.ads-software.com/support/theme/oceanwp#new-post so the theme’s developers and support community can help you with this.