What do you mean?

I mean a special training couse or exercice.

What are the keys you make such mistakes the most with? (Since you use colemak I imagine they would be: r, s, t, n, e, i, o)

No, it's not rstneio, I think it's pt, gj and rs but I have to check it.

If I may put in my two cents, those keys giving you the most trouble are the ones that need the work. Assuming that you started from the beginning courses, you'll notice that each lesson teaches you a new set of key strokes. You practice each set until you master it, then move on to the next. As you progress, the lessons will mix together so that you continuously learn each new set of keys while maintaining older sets.

In regards to those keys you're having trouble with, even if you just make up your own lesson by repeatedly typing those keys, you're bound to become good with them, i.e. rrr sss ttt nnn eee iii ooo.

The whole idea is that repetitive motion on each key will eventually become involuntary. You'll merely have to think of a letter and the muscles in your hands will automatically know which key to strike. Hope this helps!

As you progress with each lesson, you'll start to realize that certain groups of keys will have a specific key stroke pattern, such as "letter" "hand" "which". The idea behind this is so that as you read what is to be typed, your hands will automatically know that specific key stroke pattern for those words. The results of this becomes evident when you no longer read each key stroke independently, but read entire words and automatically know the specific key stroke pattern for those words. Bigger or more complex words may still require you to read each letter independently, but eventually, you'll automatically know which keys to strike to form those words. This is called muscle memory, like running, skipping, or jump rope.

For those letters that you're having trouble with, just keep practicing them.
For those words that you're having trouble with, just keep practicing them.

The bottom line is that with practice comes perfection! I've been typing for forty years and I still have letters and words that give me trouble, but with practice, you'll soon master your technique!

What is "master it". I can do a typing test without any mistake but I type slowly. Is there a speed I have to reach and should I be very comfortable with random letters like "resran". In how much time do you do this test : https://www.keyhero… with the home row and what is your accuracy?

Anyway, my main problem is bad ordering. I type the next letter before the one I should. It's not a problem with a specific letter.

Well I can't really give you much advice because I'm slower than you, but from where I am you seem to be doing well (when looking at your accuracy in your profile page), it looks to me like if you keep typing you'll get there. As for an exercise to improve your "bad ordering" problem I am going to try and suggest this to you: Write down the words you make mistakes with and then later practice on them, I don't know if this will help you or not, but if you want you can try it.
Also I have just discovered this one piece of advice: "whisper the words while you type". In my opinion this helps you to breath better, if forces you to breath more and thus get more oxygen to the brain and tissues.
Good luck, take it easy and keep at it!