I have a missing cartridge but the cartridge assembly wont go to left so I can put the cartridge in

check printer settings and ensure that the print color is ticked

Replacing Cartridge HP Officejet Pro 8600 All in One
By the way the cartridge number are 950 and 951

Here is a video from HP explaining the process. IF you followed this procedure and are getting errors, then you may have bigger issues. I am assuming you are using genuine HP ink, not remanufactured or refilled cartridges.
Replacing Cartridges for HP Officejet Pro 8600 All in One Printer Series...

Use Avery Label sheet.
Set correct paper size on Word and the Operating system.
Use label from Sheet feed tray only.

According to the printer specifications for the Officejet 8600 Pro Plus, the following label sheet sizes are supported;

HP Officejet Pro 8600 All in One Printer series N911 Manuals

Power on printer and soak up spilled ink with towels. Next we suggest you remove PRINTHEAD from printer and inspect for damage to contact pin(s) on the INSIDE OF THE PRINTHEAD ASSEMBLY. Please refer to printhead removal procedure below for reference (HP Officejet PRO 8600 Printhead Removal & Replacement Procedure):
http://h10025.www1.hp.com/ewfrf/wc/document?cc=us&lc=en&dlc=en&docname=c02871867

NOTE: To gain access to the service menus and advance printer reset procedures (and service tests) for the HP Officejet Pro 8600, 8610, 8620 or 8630 series printers please refer to our HP Diagnostics document at the following link:

Advanced HP Printer Diagnostics (i.e. printer self tests, etc.): http://inkjet411.com/?page_id=1813

Please note that if you choose to refill your ink cartridges with Costco/Frys, etc., the refill comes with a new chip so the ink level monitoring will be reset and function like a brand new HP OEM ink cartridge. We provide professional HP 950/951 ink cartridge refilling services in multiple countries and at multiple retail outlets:

INK REFILL SERVICES:
Costco USA (462 stores): http://www.costcoinkjetrefill.com
Costco Canada (57 stores, more coming soon): http://www.ca.costcoinkjetrefill.com
Costco Mexico (35 stores): http://es.costcoinkjetrefill.com/
Fry's Electronics (34 stores): http://www.frys.com/ads/page51
Best Buy Canada (13 stores): www.geeksquad.ca/inkrefill/
Micro Center (8 stores): http://www.microcenter.com/site/content/refill-service.aspx
9 Major Universities in USA & Canada
Ink Cartridge Refill Store Map Locator: http://www.go2ris.com/

Our network of retailers stand behind the product and performance - guaranteed. Customers who have questions regarding the use of refilled ink cartridges can visit http://www.inkjet411.com or contact us directly at [email protected]

Alternatively, if you need to purchase a new ink cartridge(s) and are trying to cut costs, we suggest you try a remanufactured ink cartridge from http://www.inkplicity.com/ as their ink cartridges typically have the lowest cost in the USA, have a high reliability rating, come with a 1-year warranty and provide free shipping for all orders above $20.

-Thank you

you need to replace all colors with after market cartridges not just one

just replace with new cartridge thanks

Try this once: remove the cartridges you did place, check if all contacts are clean and place them back, one by one. Just a little piece of paper or plastic can spoil the fun. Also check the con acts in the printer if you can see them. (eventually use a slash light)
Make sure all cartridges are correctly placed and every lit and lock is closed correct.
Then start again.
If tis does not help, call HP.

open front door from left side just pull outside and press and release the cratridge which is to be replaced it comes outside (acting like push to on button)
again push new one

Same problem here