Looks like appveyor might not yet have that SDK installed? The build scripts, once upon a time, would install the target SDK, but I think life turned out simpler relying on an installed version :-)
fixed it. TBH I find the use of global.json to be too restrictive for people contributing. i omit that file from my repos, and just ensure the CI server has a new enough SDK on it
Looks like appveyor might not yet have that SDK installed? The build scripts, once upon a time, would install the target SDK, but I think life turned out simpler relying on an installed version :-)