Could not create Appxupload under AppPackages folder in UWP

Because your csproj configuration may not correct configured.

Open the csproj file(project file) with text, make sure

is TRUE, that’s it.

Force a resource language in UWP

If your app published in only one language such as zh-cn, but the generated package contain two languages such as en-us, zh-cn, you should to force language in Package.appxmanifest.
Locate the codes here:

Changes to eg: zh-cn: