When using a registered domain for an nopCommerce store an SSL certificate will need to be installed on the site before SSL can be enabled. An overview for installing a certificate can be found on the following articles
To enable SSL, log into the store and then enter the Administration section of the site
Then the SSL setting can be found within Configuration > Stores > Edit
Make sure to update the "Store URL" with "https", check the "SSL enabled" box and then "Save".
If only SSL is enabled without also updating the Store URL, it will lead to redirection loops when visiting the store.
In those cases, or if SSL is accidentally enabled without a certificate being installed, either setting can be updated directly through the database. If unfamiliar with updates directly to the database, a ticket can be opened with Support