Chewing on leash

Hi folks, looking forward to visiting this site. I have a 7 month old Bernese mountain dog puppy named Reese. I got her at 4months. She is doing well with the sit, down and wait, all of which still needs to be foolproof. I now want to walk her on a leash, that's where my problem lies. . All the previous training has been done in the house. Even working on the heel which now I'm thinking I'm jumping the gun. She was doing really well off leash at the heel position just in the living room. We've worked on this for about a week now and she's really getting it ,BUT today she will walk at heel for probably 20 steps and then decides to jump on the couch and lay there. Since I haven't worked with a leash, there's no way to get her off the couch and no amount of treats is moving her. I'm stuck. I did however tried to put the leash on her but she just grabbed it, chewed it and pulled it and I just didn't know what to do. Maybe someone can help me. I'd totally appreciate it. I really want to walk her.