Plugin could not be activated because it triggered a fatal error.

Can you post the whole message? The reason about the error is always displayed right below the message.

Hi Thank you for your support.
Unfortunately there isn’t any error message displayed below, you can see attached screenshot on https://ibb.co/nbkvJA

Thanks
Alex

That’s odd indeed, the message is missing.
Can you try to enable debugging? Edit your wp-config.php script and add this line:

define( 'WP_DEBUG_LOG', true );

The error log will be written to /wp-content/debug.log.
Try again to active the plugin, then check if the error log was created and contains more details about the error.

Hi thank you for your help.
Unfortunately enabling debug log does not display any things in debug.log

In google chrome console I can see this error

Refused to display ‘https://www.site.com/wp-admin/plugins.php?action=error_scrape&plugin=ninjascanner%2Findex.php&_wpnonce=6ef6080692’ in a frame because it set multiple ‘X-Frame-Options’ headers with conflicting values (‘SAMEORIGIN, DENY’). Falling back to ‘deny’.

That means you have two applications that issue the “X-Frame-Options” header.
Did you enable if from NinjaFirewall “Firewall Policies > Advanced Policies > HTTP response headers” and from another security plugin (or at the server level)?
Make sure only one instance is sent, and that it is set to SAMEORIGIN.

Sorry I dont really understand what does it mean but
in my funtion.php i have

function send_frame_options_header() {
@header( ‘X-Frame-Options: SAMEORIGIN’ );
}

I have also disable all plugins, uninstalled some of plugins including ninja firewall, but the issue still occur.

Did you check your .htaccess? Some plugins may add the X-Frame-Options to the main .htaccess.
You can also check from online websites to see all headers returned by your server: https://securityheaders.com/?q=&hide=on&followRedirects=on
Note that this will return headers for the frontpage (unauthenticated), not the backend (authenticated).

Thank you very much for your help!

The issue was in my nginx configuration on cipherlist.conf
add_header X-Frame-Options DENY; I just commented this line and activated ninja scanner successfully.

Cheers
Alex