Need help with importing Camera Roll images from iPhone to Mac
If I understand your question correctly, it sounds like your Mac is simply asking you to unlock your iPhone. With an iPhone 6s you would do that by entering your Passcode or using Touch ID.

In summary, to use Image Capture the typical process would be:


On your Mac launch the Image Capture app
Connect a USB/Lightning cable to your Mac
Plug the other end of the cable into your iPhone 6s
At this point you should be asked to unlock your iPhone and/or to Trust this Computer - enter your Passcode or use Touch ID and, if prompted, tap on the Trust button
Within Image Capture select your iPhone 6s from the Devices list
You should now see a list or thumbnails of the photos on your iPhone 6s
Select the images you want to import (or select nothing to import all photos not previously imported - this could be a large number) 
Click on the Import to drop-down menu to select where you want the photos saved to


You can also set up Image Capture to automatically import photos from your iPhone 6s. For more information, refer to Image Capture Help: Transfer images.

As for iPhoto (or the Photos app), your Mac would still be asking you for the same thing - So the broad process described above will also work for those apps.



UPDATE due to additional info provided in comments

An iPhone 6s with iOS 11.x installed requires the following specs for syncing:


Mac: OS X 10.9.5 or later
PC: Windows 7 or later
iTunes 12.5 or later 


Based on this, your 2011 MacBook Pro isn't capable of syncing in general with iTunes while it still has Snow Leopard installed and a version of iTunes earlier than v12.5. 

However, you should still be able to import photos using Image Capture v6.0.1 (the version you'd have installed with OS X 10.6.8).

As a next step, I would try creating a new user and testing with that.

Create another user

In Snow Leopard (i.e. OS X 10.6.8) you can create a new user in Apple > System Preferences > Accounts and then log into that user to see if the problem persists. 

You can also try testing to see if you can import photos via booted into Safe Mode.

As a troubleshooting step I would boot your Mac into Safe Mode.

Boot into Safe Mode

Follow these steps to boot your Mac into Safe Mode:


Fully shut down your Mac
Restart your Mac
Immediately press the Shift key and keep it down 
Let go of the Shift key when you see the login window (NOTE: If you have FileVault enabled you may need to log in twice).
Take a note of what happens (i.e. try using Image Capture)
Exit Safe Mode by restarting your Mac as normal
Log in and test Image Capture again


Let me know how you go.