I previously installed 2018 version by mistake, then I deleted it because my software only can run on 2015 version. While when I install 2015 version, it said unable to install:
NI Variable Engine (higher version already installed)
DataSocket (higher version already installed)
NI USI (higher version already installed)
I check the control panel, the 2018 is not installed.
Any idea of how I can uninstalled these?
Did it install LabVIEW 2015 or not?
If newer versions of some shared components are already installed, they won't get downgraded, so that's more of an informational message than an error. They won't get installed because they already are.
Newer Versions of LabVIEW (such as LabVIEW 2018) must be correctly uninstalled before installing an older version (such as LabVIEW 2015). If you look at today's Posts, you'll find another discussing removing LabVIEW 2017 (or 2018, don't remember which) that has some links I posted discussing the safe way to remove LabVIEW. Note that trying to do it by "brute force" can, in some cases, lead to corrupting your Windows installation (it happened to me on two different machines).
I uninstalled the file in the control panel, and then deleted the whole folder. That is it.....
It seems that still has some leftovers.....
You will find the links in the middle of this thread. However, if you have (as you've described) "deleted folders", then you absolutely should waste no more time in trying to "repair" -- you need to "rebuild". Copy all of your documents and other important files to an external drive. Make a list of all installed software. Reformat your C: drive and reinstall Windows 10. Reinstall all your other software. Reinstall your documents. Note that (other than the time spent gathering installation media) this whole process should take no more than 2-4 hours, leaving you with a "clean machine" that works. I know (from my own sad experience, with >10 years LabVIEW experience and maybe >100 installations on various machines) that once NIPM entered the picture, trying to "fix things" can take literally weeks before you wise up and re-build your machine.
Once you are ready to "install LabVIEW for the first time", remember the rules -- install the oldest Version first, install device drivers last, and (my personal advice!) don't install "future updates" (i.e. if running LabVIEW 2015, don't install updates and patches with numbers like "17" or "2017"). Be especially careful about crossing the NIPM/LabVIEW 2017 boundary -- once you do, uninstalling become much more "dangerous".