Source build of 4.21 failing to build with all these weird errors

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I’d focus on the first error in the logs:

f:\ue\ue 4.21\engine\source\developer\directorywatcher\private\DirectoryWatcherPrivate.h(19): fatal error C1189: #error:  "Unknown platform"

Looking at that code it appears that it can’t detect what OS you are building on. Or the OS you are building on is not Windows, Mac or Linux.

If it can’t detect that then it makes sense that everything would break. Are you attempting to build on an OS that is not one of those three?