Dog won’t sit on hard surfaces

Hi! Recently adopted a 1yr old chihuahua pug mix who hasn't had any training at all. Have been trying to teach him to sit, but he refuses to sit on hard surfaces (outside, kitchen tiles etc). He understands the command and will do it immediately if he's on a rug/the couch/towel on the floor, but wondering if anyone has any tips about getting him to be comfortable sitting on any surface?

Thank you 🙂