The OpenJPEG build error as part of the ITK project is because you’re likely using Visual Studio 2019 version 16.9 or later. This issue was fixed in this commit, but that means you will need to build latest Slicer master branch to have that included. You were building Slicer 4.11.20200930 which is quite old and even older than the current Slicer 4.11.20210226.