What's wrong on my code?
You can't focus controls in OnCreate event. Put your code in OnShow event.
I did it alreay,but the same error message had been released
It doesn't look like you did that in your original post. You try to focus Memo1 in method named FormCreate, which is default when you double click on form or if you click on OnCreate event in object inspector.
I think you didn't do put your code in OnShow event properly. I mean: if you get this error, then focusing in OnCreate is still happening.
I suggest you make small project that shows this behaviour and post it here.