Why I can't activate my iphone 3gs after restore using itunes?

Do you have anti-virus software running on your computer? Sounds like a disconnect in the internet connection necessary to contact the activation server. You can also take a look at this support document and see if anything there will help. If you can't activate your iPhone - Apple Support

You cannot activate an iPhone without a SIM installed.

I'm using a MacBook Pro

Running macOS Sierra 10.12 Developer Beta 3

Running macOS Sierra 10.12 Developer Beta 3

Okay, never saw Mac indicated, but see the support document that I linked.

This above could also be your problem too.