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

What happens when you try? Do you have a SIM in it? Was it ever jailbroken or hacked to unlock it?

I'm having the same problem and my device is not jailbroken or anything...

What happens when you try to activate? Do you get any messages?

Yes. It says "Your iPhone could not be activated because the activation server is temporarily unavailable. Try connecting to iTunes to activate it, or try again in a few minutes"

I have been trying for two days now... I've tried everything I found on google.

Was this iPhone previously jailbroken or rolled back? Did you try to activate it on a different computer?

No. and yes

If it didn't work on another computer and this is an iPhone 3GS I'm going to guess that at some point it was hacked to unlock it. Try placing it in DFU mode (Google it) and then Restore it as a new device. If you are using a Windows machine temporarily disable ALL antivirus/antispyware software and do so. In your Google searches did you find a suggestion regarding gs.apple.com?

How do I restore it if its in dfu mode?

Hard reset, DFU, recovery mode

Ok I will do it now.

Nope its still got that problem.

It also says "no service" or "searching" in the top left corner of the display...

I forgot to mention it before... I also tried to take out the SIM card and it still says that...

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