Most likely Anaconda messes up your build environment. You need to temporarily rename your main anaconda or conda folder to something else, build Slicer, and you can then restore the original name.
Thanks! I now just have one error:
Severity Code Description Project File Line Suppression State
Error MSB8066 Custom build for 'D:\D\S4R\CMakeFiles\551ca37c0026cda4fee29f0729cb6475\python-update.rule;D:\D\S4R\CMakeFiles\551ca37c0026cda4fee29f0729cb6475\python-patch.rule;D:\D\S4R\CMakeFiles\551ca37c0026cda4fee29f0729cb6475\python-configure.rule;D:\D\S4R\CMakeFiles\551ca37c0026cda4fee29f0729cb6475\python-build.rule;D:\D\S4R\CMakeFiles\551ca37c0026cda4fee29f0729cb6475\python-install.rule;D:\D\S4R\CMakeFiles\551ca37c0026cda4fee29f0729cb6475\python-configure_python_launcher.rule;D:\D\S4R\CMakeFiles\3b657cd1f5796cbed5301f0e73ca6eec\python-complete.rule;D:\D\S4R\CMakeFiles\0b4a24838f508b9d1726cbe947087ed5\python.rule' exited with code 1. python C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Microsoft\VC\v160\Microsoft.CppCommon.targets 241