Hi K.,
I also started putting my son in time out at 18 months for not listening. He was continually climbing up our couch and its a long fall to the bottom. He didn't get it at first, but after doing it for a week or so, he finally started understanding this was not the place he wanted to be. I just gave him a warning that timeout was coming if he didn't do whatever it was I was asking. If he did it again I put him in there and set the timer for a minute. After I took him out I explained why he was in there and we went on to another activity.
He is no longer climbing on the couch so he hasn't been in timeout for a little while. I have also been putting him in there if he is having a hard time calming down (i.e. tantrum). I don't throw him in there for every little thing. I try talking him out of things first, even tantrums by verbalizing his feelings for him and that has been helping a lot.
Keep at it and she will start to understand eventually.
Hope this helps!