There might be a hacking issue on your website, you can do the following
1. check website through any site checker tools (to check if there is malware, blacklisting status, website errors or out-of-date CMS)
2. Update all login access (Cpanel, CMS, FTP)
3. fix the issues either by removing the script or plugins that causes this
4. then mark as "fixed" in Google Search Console if you believe that you have removed this problem