Not allowed to access this page

Code Snippets checks for the manage_options capability when determining who can access the plugin. Resetting the administrator role so it once again has that capability would perhaps be the best way to fix this.

OK thank you!

But if I wanted the codes of a “Snippets” page to be active only for a user role what should I write?
Thank you

You can reassign the capability that Code Snippets checks for using a snippet:

add_filer( 'code_snippets_cap', function () {
	return 'some_capability';
} );

Just need to replace some_capability with the capability you want to use, either one that comes with WordPress or a custom one you have created.

for example if you wanted to insert some css in admin,
should I write like this?
Thank you

add_filer( 'code_snippets_cap', function () {
	return 'some_capability';
} );
add_action( 'admin_head', function () { ?>

<style>

The snippet I posted doesn’t need to be included in everything you write. Add it as its own snippet, and it will work for the entire site.

sorry for me it’s not clear yet.
What I try to do is activate some snippets by user role only.

Oh, I see. Okay, here’s an example you might use if you wanted to add some CSS to the admin area for a particular capability:

add_action( 'admin_head', function () {
	if ( ! current_user_can( 'do_something' ) return;

	?>
	<style>
		
	/* CSS code goes here */

	</style>
	<?php
} );

Replace do_something with the relevant capability. If you would prefer to check based on role instead, see my answer to this question.

Thank you!