B737 CREW DEPARTING LAS CAN'T FOLLOW RNAV DEP DUE TO MIS-PROGRAMMING OF FMC.

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DURING TAXI OUT FOR DEP; I NOTICED THAT THE DEP RWY WAS NOT LISTED UNDER THE TKOF SPDS FOR UPDATE. I CHKED THE PREFLT AND EVERYTHING LOOKED CORRECT AND AFTER DISCUSSING WITH THE CAPT I THOUGHT IT WAS A GLITCH DUE TO OUR HAVING PREVIOUSLY RETURNING TO THE GATE AND HAVING SEVERAL PWR SWAPS WITH OUR MAINT PERFORMED. WE WERE IN A LONG LINE OF ACFT WAITING FOR DEP AND IT HAD BEEN APPROX 30 MINS SINCE WE PUSHED THE SECOND TIME. I THOUGHT ONCE WE WERE AIRBORNE WE WOULD GET A POS UPDATE. AFTER TAKING OFF; THE FMC DID NOT PROVIDE GUIDANCE TO RBELL AND IT WOULD NOT ACCEPT A WAYPOINT CHANGE; SO WE WERE MANUALLY STEERING TOWARDS THE WAYPOINT AND AS THE FMC COUNTED DOWN TO THE WAYPOINT WE WERE GOING TO MANUALLY TURN. THE FMC WOULD NOT ACCEPT A LEG CHANGE TO ANY OF THE WAYPOINTS ON THE COWBY 2 DEP. AS WE STARTED THE TURN LAS VEGAS DEP ASKED IF WE WERE HAVING DIFFICULTY WITH OUR RNAV AND WE SAID WE WERE AND WE ASKED FOR RADAR VECTORS. AT THIS TIME THE CAPT TOOK CTL OF THE ACFT AND FLEW THE ACFT WHILE I WORKED ON THE FMC. THE FMC FINALLY ACCEPTED GUP VOR ON THE FLT PLAN AND THE FMC WORKED NORMALLY FROM THAT POINT ON. AFTER REVIEWING WHAT HAPPENED OVERNIGHT; I CAME TO THE CONCLUSION THAT I HAD IMPROPERLY ENTERED THE FLT PLAN. WHEN WE ARRIVED INTO LAS ON THE PREVIOUS LEG WE WERE APPROX 15 MINS EARLY. I DID NOT GET THE OUTBOUND PAPERWORK FOR BTWN 5-10 MINS. WHEN I GOT THE PAPERWORK; I STARTED ENTERING PERFORMANCE DATA; INCLUDING OUR FLT LEVEL; WHEN I REALIZED THAT I HAD FORGOTTEN TO ENTER AND ACTIVATE THE FLT RTE. WHEN I WENT TO THE FMC PAGE TO ENTER THE FLT RTE IT GAVE A MESSAGE OF AN ALT CONSTRAINT FOR THE FIRST XING RESTR. I HAD ASSUMED THIS WAS BECAUSE I HAD FIRST ENTERED THE FILED FLT LEVEL. I CLRED THIS AND THOUGHT I HAD ACTIVATED THE FLT PLAN. NOW THAT I THINK BACK I BELIEVE THE FLT PLAN WAS NEVER ACTIVATED. SINCE WE PUSHED OUT AND THEN RETURNED FOR MAINT; I HAD ASSUMED THE PROB AROSE FROM THE NUMEROUS PWR CHANGES WHEN THE MAINT WAS PERFORMED. I SHOULD HAVE BEEN MORE DILIGENT IN PERFORMING MY DUTIES; EVEN IF IT REQUIRED REALIGNING AND RE-ENTERING ALL THE DATA. BEING LATE AND WAITING IN A LONG LINE FOR DEP PRIOR TO NOTICING THE ERROR CAUSED ME TO THINK IT WOULD BE ALRIGHT; SINCE EVERYTHING LOOKED LIKE IT WAS ENTERED PROPERLY.