Remove empty top bar above header

This can certainly be solved via CSS. Since there are thousands of themes, and also many individual ways to create a design, a link to your site would be important if one should be able to help you here.

I see several ways you can solve this:

a) With this custom css entry

body > .wp-site-blocks {
  padding-top: 0;
}

With the theme you chose, there is (to my knowledge) no way to add this without another plugin. You could use this plugin for this: https://ww.wp.xz.cn/plugins/custom-css-js/

b) You set the root paddings in TwentyTwentyThree so that the top value is 0. Details: https://make.ww.wp.xz.cn/themes/2022/09/07/full-width-blocks-and-root-padding-in-wordpress-6-1/

c) You ask this in the support forum of the TwentyTwentyThree theme you are using: https://ww.wp.xz.cn/support/theme/twentytwentythree/ – they will be able to give you a much more precise answer to b).