This message pops up when I was upgrading a Windows 7 machine to the latest Windows 10 the other day, telling me that:
The program can't start because api-ms-win-core-libraryloader-l1-1-1.dll is missing from your computer. Try reinstalling the program to fix this problem.
If you ended up reading this post, you are probably having the same issue as I did. If so, here is how you can fix it. It’s pretty easy, only if you know where to look.
The problem is that the upgrade process is looking for the wrong DLL file. The api-ms-win-core-libraryloader-l1-1-1.dll which appears in the error message is for Windows 8. The one for Windows 7 is one version lower like api-ms-win-core-libraryloader-l1-1-0.dll. By acknowledging that, the fix is pretty easy to follow.
All you need to do is replace the wimgapi.dll file in C:\Windows10Upgrade folder with the one in C:\Windows\System32 folder.
Open File Explorer, navigate to C:\Windows\System32 folder, locate the file wimgapi.dll and copy/paste to C:\Windows10Upgrade folder.
Or, simply run the following copy command in an elevated Command Prompt window.
copy wimgapi.dll c:\windows10upgrade
Once done, either click Try again button if you haven’t canceled the upgrade or restart the Windows 10 upgrade process. Since the download part is already done, it goes pretty quickly to the installation process.
Good luck and hope it helps.