I'm having a problem to create a PowerApp to use on mobile. I've got my EditForm with fields from SharePoint list. Below that fields I need to have a button. The problem is that it doesn't fit on the screen any more and I don't how could I add a scrollbar to be able to see it.
By the sounds of it what you need is a "Fluid Grid" control. Unfortunately, there is no way to add these in a straight-forward way to existing screens - but there is a work around.
Select New Screen > Scrollable Screen from the Home or Insert menus.
As you would think, this adds a new screen. In this screen will be a control called Canvas1 (or similar). This is the "Fluid Grid" control you want. Copy this control. (I find using Ctrl + C can be a bit unreliable so I right-click and copy from the left-hand navigation panel shown below)
Go back to your original screen and paste the control there. (Again, I right-click paste in the navigation panel). The canvas and it's first blank datacard will now be on your existing screen.
Adjust the X, Y, Height and Width properties of your new canvas so it covers the screen less your header bar (if required)
Now select everything that you want to have scrollable on the page (your edit form, submit button etc. but not your header bar, dilaogs and the like) and cut it from the screen.
Select the blank datacard under the canvas and paste your items.
You should now have a screen that is scrollable just like below. You may need to readjust the positioning of items if they are static.
It's not the most straightforward method but it has reliably worked for me before.
Of course, the best way to get a scrollable screen is to build it that way initially using the New Screen > Scrollable Screen option. This might not always be an option however such as if you are using the tool to automatically generate an app from a datasource.