I ran a quick search on Internet with the text that appears in that error message [1] and found several pages describing the same error, first of all this is not related to the Sucuri plugin, and not even WordPress, this issue is caused by the server where the website is being hosted because the SSL certificates that are used during the execution of a HTTP request (for example with CURL) are not valid, this usually happens when the server has self-signed or expired certificates.
I recommend you to communicate with the support of your hosting provider and resolve this issue with them. Or you can go to the plugin’ settings page and disable the “API request with SSL”, this will force the Sucuri plugin to send its HTTP requests without verify the SSL peers.
[1] https://www.google.com/search?q=%22SSL+peer+certificate+or+SSH+remote+key+was+not+OK%22
