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All I've been doing is adding a "/" in the htaccess field entititled "Force HTTPS for these URLs (do not include the domain name)"

Do not do that. This is not what this feature is designed for. This feature was designed to force accessing specific URLs through HTTPS, but not the domain root, on an otherwise HTTP-only site. Think about an e-commerce site that runs on HTTP but the checkout needs to always be on HTTPS. This feature is NOT designed to make your site HTTPS-only.

The correct way to do what you are trying to accomplish is setting the Joomla Global Configuration "Force SSL" to "Entire site". Before doing that make sure that "Force HTTPS for these URLs (do not include the domain name)" is empty. After you confirm your site works fine under HTTPS go to the .htaccess Maker and enable the HSTS feature. This will tell browsers to never bother accessing your site under plain HTTP.

and changing "HSTS Header (for HTTPS-only sites)" to "yes"

Only do that AFTER making sure that you can access your site through HTTPS!

When I try to use the Joomla global configuration to force https on the site it comes back with the message that https is not available on the site, which is incorrect.

You made a mistake by prematurely enabling HSTS. Please clear the HSTS cache. Remove the "Force HTTPS for these URLs (do not include the domain name)" and HSTS settings from the .htaccess Maker and create a new .htaccess file. Try accessing your site over HTTPS. If this still fails, contact your host. If it works, see above about what you should do.

You're welcome, David!