Toilet training can be hard, but here's how I got my 2 to learn.
We have a glass sliding back door leading to the backyard, so everytime we take them out to go to the toilet (after playing, sleeping, eating) we would put them on the ground and tap the door with one of their front paws. Then once we were outside, I would tell them to go "on the grass".
Now, everytime we are inside and they want to go out, they will just tap the back door with their paw. Or if I'm taking them out before we go to bed, I just tell them to go "on the grass" and off they run.
If you have a noisy house, you could also put a bell near the door and get them to tap that.