In previous versions of the OS in the security pane, you are provided an option of:
But in this version there is no option anywhere:
Is there a way that I can turn on anywhere? (via Terminal maybe?)
Apple have removed this function on macOS Sierra, but you can re-enable it running this in terminal:
sudo spctl --master-disable
Additionally, as Jack points out:
Granted this is a little overkill, but it does the job reliably. It can be re-enabled immediately after launching the application with
sudo spctl --master-enable
OS X improves download validation by providing file quarantine in applications that download files from the Internet. This means that downloads are checked for safety (known malware) when you try to open them.
You can check an app (or any file) in quarantine using terminal. It's literally an extended file system attribute:
mac:~ user$ xattr /Applications/Some.app com.apple.quarantine mac:~ user$
If you're absolutely sure it's safe to run the app, you can drop the attribute like follows (you'll be asked to provide your password):
mac:~ user$ sudo xattr -r -d com.apple.quarantine /Applications/Some.app
Check once again:
mac:~ user$ xattr /Applications/Some.app mac:~ user$
Now you should be able to run the app.
Gatekeeper options are redesigned by Apple for Sierra.
What it comes down to is that Apple is removing the "Install from Anywhere" option for the "general user" or the masses of people that are not power users.
However as already answered by user Mateusz Szlosek, you can still open unsigned apps via Finder by right clicking (Ctrl clicking) on the Application and selecting Open.
If the app was extracted from an archive, it might be your archive extraction utility.
After checking out the other answers found here, none of them were working for me.
Eventually I realized the issue was my archive extraction utility (Keka 1.0.6) was silently failing to extract the app from its .zip file, and even though the app looked intact with the correct icon and everything, it was failing to read the package contents and would throw up a generic error message:
The application “AppNameHere.app” can’t be opened.
I swapped back over to using the built-in "Archive Utility.app" and everything went smoothly after that. So if
sudo spctl --master-disable doesn't work and
xattr shows the file isn't quarantined, try a different extractor utility.