The dialogue box above did not give sufficient information about which file it could not modify – but it is apparently part of the unpacked install files.
The problem was nothing to do with Administrator privileges.
The advice I received from Autodesk was to do a full uninstall of Revit 2020, and then reinstall. It turns out that may not have been necessary – but it did eventually work . . . .
Hard Disk Space
My computer has a solid state drive (SSD), which used to be very expensive, hence it is only 200Gb. That means that disk space is valuable – not an uncommon situation. This is apparently of no significance to Microsoft or Autodesk – every version of their software consumes more and more disk space. 200Gb gets eaten up very fast by their software, but also by their upgrade/install process.
Each time that Windows has an upgrade, it often demands at least 10 or…