I have no fields available for sorting with Search API sorts module
Search API Sorts only lists the fields of supported types. Make sure the fields you want to sort on are single value strings or numbers.

If the fields are mutli-value you can use Search API Ranges‎ to give you max/min value fields that you can use.

If a field is full-text, one trick is to create an aggregated field containing just that field and assign it a string type.