lot of people are having same activation error messages in 3gs when try to update 6.0.1. I have tried few random methods sometimes works
1) use new sim cards for activation. some time old style sim cards is not working for activation.
2) if that does not work use this method
https://www.youtube.com/watch?v=FEBSMuyUQjA
3) if anything above does't work downgrade to 4.1 is only option. let me know if you need any help in that
it worked for me if it work on yours press thanks and we are even