Disable plugin for administrator

I just tested your code and it works, so it seems one of the two following things.

You are placing the code in the wrong place, make sure you put that code at the bottom of the functions.php file of your theme. Or maybe the functions.php file of your theme is having problems, in which case you could share it with us by posting its content editing your question.

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It seems alright, do this:

Open your wp-config.php and add these lines:

define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );
define( 'WP_DEBUG_DISPLAY', true );

Let me know what it shows after.