I want to put different note lengths in one bar above or below each other but when I change a note in guitar pro then other notes change too, so I have to use ties for other notes and that's very nerve-racking Do you know any way to solve this problem ?
You should look into using multiple voices (find a similar article for your GP version) - but make sure it's still playable. If it isn't playable, you should look into using multiple instruments.
To summarise the article, here's a screenshot showing what I mean (complete with some handy freehand circles). My screenshot is of Guitar Pro 7 so the icons might be in a different place in your version's UI, but they should look the same (or similar enough):
To switch between voices, you click on one of the numbers circled in red. You can have up to four different voices.
To view all four voices at the same time, so all of the notes are a solid black and not greyed out like in my screenshot, toggle the button circled in yellow. I personally like to only work on one voice at a time when writing music so tend to leave it toggled off, but if I want to see how readable my music is as a whole (ie the bar lines don't cross, it doesn't look too cramped, etc) then I'll toggle this.