Q: How do I download my photos from my iphone6s to my mac running 10.11.4?

You have three different issues here.

as per the iphone.

If you are having problems with apps stalling or crashing,

-- double click on home button

-- swipe up on all the apps to force quit them

Re-boot ipad.

Press and hold the Sleep/Wake button until a red slider appears, then drag the slider. Then press and hold the Sleep/Wake button until the Apple logo appears. 

https://support.apple.com/en-us/HT201559

force power down if the above give you trouble.

Hold down both the Sleep/Wake button and the Home button. Ten seconds or more.  Hold until the Apple logo appears.

more details on re-booting:

    http://ipad.about.com/od/iPad_Guide/ss/How-To-Reboot-Your-iPad-Power-Off.htm

This these tips by MichelPM to improve performance:

https://discussions.apple.com/message/29504169#29504169

https://discussions.apple.com/message/29504311#29504311

Restore to factory settings by loading the latest version of ios that your machine will support:

   1)  Do a full backup of your device if needed.  Twice is best -- cloud and computer.

       https://support.apple.com/en-us/HT203977

   2) Three ways of restoring your ipad.  [ as easy as abc ]

   2.a) "You can use iTunes to restore your iPhone, iPad, or iPod touch to factory settings and the latest version of iOS."

          http://support.apple.com/kb/HT1414?viewlocale=en_US&locale=en_US

   2.b) Be a little more insistent:  "You can put your iOS device into recovery mode, then restore it using iTunes.:

          http://support.apple.com/en-us/HT201263

   3.c) At last resort you could consider DFU (Device Firmware Update).  This could void your warranty.

          http://www.imore.com/tip-put-iphone-ipad-dfu-mode

You have multiple questions about multiple prorgams

as to this forum, Photos for Mac, you are doing to have to provide details - "unable to download photos" gives us no clue as to what is going on or wha tyou are doing - so does "I just get error messages"

The reason that error messages are displayed is to provide information so people can help so obviously it is critical to provide them to us - along with the software versions you are using

and what has changed since everything worked?

as to the migration typically that would mean you just (finally after over a year) upgraded your OS and added the Photos application to your system - the first time you launch it is migrates your last used iPhoto library to Photos --    How Photos handles content and metadata from iPhoto and Aperture - Apple Support   --       and the most common reason that everything is not there is that you have multiple iPhoto libraries and need to migrate each one - to migrate additional libraries drag them to the Photos icon in the Dock

as to iTunes you need to ask in the iTunes forum - and again they can not see you so you need to provide details there too

for assistance is writing a question which can be answered see  Writing an effective Apple Support Communities question

LN