Since you have subscribed to the yearly plan, you will get the carrier calculated rates feature for free. You have to contact Shopify support to enable this in your store. (You can contact them via chat for a faster response)
As @HariKrishna mentioned, if you need to enable carrier calculated shipping (CCS) on your account, you'll need to reach out to our Support team directly since we can't provide account-specific support on the Community at this time. If you're already on an annual billing plan, our team will simply need to add the CCS feature for you.
If you're having troubles connecting with our team, can you please share a screenshot of what you are seeing? We can help you through live chat, callback request, or email. You can click here to contact our Support team. Simply log in and follow the prompts to contact Support.
Furthermore, as you mentioned you are having an issue with an app, I recommend reaching out to the app developers directly to receive troubleshooting assistance if you haven't already. Their contact details can be found from the SEFL LTL Freight Quotes app page in the Shopify App Store under Support.