Andrew,
Your canonical needs to exist - so for the page you mentioned you need to update the url to the one with the trailing slash. In fact - for all pages on your site you should check if the canonical exist (Screaming Frog can do miracles here)
The issue with the https is a bit different - you should not have both versions (http/https in parallel) - so if your https certificate is ok you should put your site in https & redirect the http version to https
This can be done by adding these lines to your htaccess:
RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI}
If you switch to https - you must make sure that all the resources you call on a page are also on HTTPS - if not, users could get a security warning. There is an article on how to migrate site to HTTPS on Yoast: https://yoast.com/move-website-https-ssl/
If your site is on https - your canonicals need to be in https as well. A tool like Screaming Frog can help you to check that both (https & canonicals are ok) - it's not free - but certainly worth the investment.
Hope this helps - don't hesitate to ask if it's not clear
Dirk