I made a farmhouse table and stained it with Rust-oleum water based stain and then use the Varathane Triple Thick Polyurethane on top. I have let the table sit out in the garage for a week to cure, but I case still scratch it with my finger nail, like hard wax. Is this normal or will it cure harder? Is this a bad Polyurethane to use?
Assuming 70°F, 7 days to cure. You stated your temp has been 40°-60° and did not state humidity. There is nothing wrong with the product, you have used it outside of parameters recommended by the manufacturer (see text below). Take it into a temperature and humidity controlled space and hopefully it will cure hard.
Dry times are based on 70°F and 50% relative humidity. Dry times may be affected by temperature, humidity and ventilation. Allow more time at cooler temperatures. Triple Thick Polyurethane will be dry to touch in approximately 2 hours and can be recoated in 2 hours. Allow 48 hours cure before light use of the project. Allow 7 days for full cure.