Narrative:

[After the flight I was told that] den TRACON had a question about our flight into den. We had flown the peekk 3 RNAV arrival into den and were told to expect runway 16R. As pilot monitoring; I had loaded RNAV Y 16R into the FMC. We also had the localizer frequency to 16R loaded in the navigation radios.while we were inbound to clfff on the arrival; den approach cleared us the RNAV Z 16R and to start at clfff. I immediately loaded the RNAV Z 16R into the FMC via the arrival page and chose the clfff transition. However; the FMC would not allow me to execute the change as we were about on top of clfff already. (Clfff is a waypoint on the arrival as well as an if (initial fix) on the approach.) we tried to load the approach one more time but still could not get the new approach to execute. On both attempts I did page through the legs pages to get the proper clfff to load at the top of legs page one; but it would not execute.we could see the arc and the approach on the navigation display and decided to turn off the autopilot and hand fly the approach since the autopilot was continuing us straight ahead on the arrival towards greys. The captain flew the aircraft well and stayed on speed and on course or very close to the course the whole time. I monitored everything and did not notice any navigation performance deviations (never saw anp bar limits). We did see company traffic on approach to 16L and made sure we were well west of them. That probably put us inside the final arc to jetsn. At no time did we feel we lost situational awareness or put the aircraft in a dangerous situation.in hindsight; after I could not get the FMC to execute the new approach; I should have let approach know that we were 'unable' to fly the approach and requested vectors to final or a visual approach instead. I would recommend to TRACON that they issue approach clearances as early as possible in order for flight crews to load them in a FMC. We are all human and I think that two extra minutes would have made the difference in this instance.

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Original NASA ASRS Text

Title: B737 First Officer reported that they deviated from clearance due to flight management system not accepting new arrival.

Narrative: [After the flight I was told that] DEN TRACON had a question about our flight into DEN. We had flown the PEEKK 3 RNAV Arrival into DEN and were told to expect Runway 16R. As Pilot Monitoring; I had loaded RNAV Y 16R into the FMC. We also had the Localizer Frequency to 16R loaded in the NAV radios.While we were inbound to CLFFF on the arrival; DEN Approach cleared us the RNAV Z 16R and to start at CLFFF. I immediately loaded the RNAV Z 16R into the FMC via the Arrival page and chose the CLFFF transition. However; the FMC would not allow me to execute the change as we were about on top of CLFFF already. (CLFFF is a waypoint on the arrival as well as an IF (Initial Fix) on the approach.) We tried to load the approach one more time but still could not get the new approach to execute. On both attempts I did page through the LEGS pages to get the proper CLFFF to load at the top of LEGS page one; but it would not execute.We could see the arc and the approach on the NAV Display and decided to turn off the autopilot and hand fly the approach since the autopilot was continuing us straight ahead on the arrival towards GREYS. The Captain flew the aircraft well and stayed on speed and on course or very close to the course the whole time. I monitored everything and did not notice any NAV Performance deviations (never saw ANP Bar Limits). We did see Company traffic on approach to 16L and made sure we were well west of them. That probably put us inside the final arc to JETSN. At no time did we feel we lost situational awareness or put the aircraft in a dangerous situation.In hindsight; after I could not get the FMC to execute the new approach; I should have let Approach know that we were 'unable' to fly the approach and requested vectors to final or a visual approach instead. I would recommend to TRACON that they issue approach clearances as early as possible in order for flight crews to load them in a FMC. We are all human and I think that two extra minutes would have made the difference in this instance.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.