If your Setup.exe sags, is hanging low or even shaky, the brackets are set too wide for the thickness of your 19” rail.
To adjust it, loosen both nuts on one side with a wrench (13mm or 1/2″). You can now slide the inner part of the brackets and adjust it to your 19“ rail.
With the loosened nuts, slide the Setup.exe brackets over the 19“ rail, push the brackets together by hand, and tighten the nuts. The brackets should sit firm on the 19“ rail but can have some leeway. Remove the Setup.exe from the 19“ rail and then further tighten the nuts again.