Error in Qt Creator: "The environment variable ExtensionSdkDir is not set"

I am trying to build my project using the Visual Studio compiler in Qt Creator, but whenever I run the program the following error appears:

qt.winrtrunner: Using the Appx profile.
qt.winrtrunner: The environment variable ExtensionSdkDir is not set.
qt.winrtrunner: The directory "" does not exist.

EDIT: I have tried to add a "ExtensionSdkDir" in the build environment setting it to "C:\Program Files (x86)\Microsoft SDKs\Windows\v8.1\ExtensionSDKs". Then the following error occurs

qt.winrtrunner: Using the Appx profile.
qt.winrtrunner: Failed to activate application: 0x80073cf1 "Package was not found."

Alright, I found the solution to the problem. I set "ExtensionSdkDir" in the build environment to "C:\Program Files (x86)\Microsoft SDKs\Windows\v8.1\ExtensionSDKs". Then I noticed that I did not enable "Developer Mode" in Windows 10 Settings (Update and security), enabling this function resolved the issue.