For information, I had the same issue.
The solution I found is to remove the ReadOnly flag from RunMutableCommandlet/obj files
Then, to ensure every thing is OK, right click on Solution in VisualStudio and press “Restore Nuget Package”. It should echo “All packages are already installed and there is nothing to restore.”
My Feeling:
the nuget management feel it may “double check” that every thing is up to date but early fail as the file are ReadOnly. In a way of “Ok! It would be impossible for me to make the update as the files are ReadOnly!”. Even, in reality there is nothing to update.