This usually means that your website is hacked and Google has marked it Not secure. Sometimes even when you fix the hack, the error still shows up. The number one reason for this error is that your website is hacked and is believed to be distributing a malware. This code spreads itself from the infected website to website visitors and can even spread to other websites.
Google marks a website with this warning if they find any suspicious code that could be a malware or trojan or any unwanted program. This warns users to be careful when visiting these sites.