I have a 2012 kia optima and the obd code is a p0087 low fuel pressure on fuel rail what could be the cause of this issue

Most likely Fuel pump or filter. Might want to consider taking an actual reading with a gauge, to see what the psi is, and not just go by the OBD scan Go here and read entire page for a ton of information: http://www.obd-codes.com/p0087

Kia issued a couple of TSB's (Technical Service Bulletins) on the 2012 Kia Optima about fuel pressure issues. Depending on what engine you have, it could be the high-pressure rail, which is a tremendous amount of pressure being injected to overcome compression. You can't use a standard gauge for that. anyway see: http://www.edmunds.com/car-maintenance/tsb.html?year=2012&make=&model=&styleId=101415932&engCode=4ITCG2.0&transCode=AUTOMATIC&mileage=6000&zip=91316&type=tsb

The low side from fuel pump spec for fuel pressure is 45 +/- 5 psi , then has a secondary high-pressure pump and regulator to step it up. WAY up in to the thousands of psi. I found conflicting information about what the high pressure spec is but lowest was 2500 psi all the way beyond 4000 psi. IF you decide to work on this car, a word of caution> Bleed fuel lines to reduce pressure. A pin-hole leak of fuel will pass right through your hand. Or arm. I have seen it happen, the guy almost lost his hand. Also remember you have a 5 year 60,000 mile warranty

Try fuel pressure sensor. On fuel rail it's about a $50 part.. DON'T go to dealership