Originally Posted by Jigone

As stated in another post, it's not an error you are experiencing, it's a deliberate system function/limitation created by Shimano to prevent the R derailer from exceeding chain wrap. They only allow certain cranks and ring combo's to what they are currently selling on road groups, as well as certain cassettes. I was actually surprised to learn (when I installed the 8050 group) that they've discontinued the Ultegra 11-23 cassette (which I've used for years), with the only explanation is they assume the user can get by with the more limited selections.

You CAN make the system function the way you want if using the entire 8050 group, by deselecting Gear Position. To do that you have to tell the system that you have the 36/46 or 39/53 cranks in use and are in Manual shift mode, then it's good to go with all cogs with both rings.

Caveat 1 on this is if desiring to have your ring and cog displayed on a Garmin or similar device, then having chosen a crank and/or cassette that is different then what the system thinks you are using, the ring/cog combo will display incorrectly. Not a huge issue for many. Caveat 2 is that the full Synchro mode system has limitations as to how many cogs it will shift across so essentially and by choosing the shift points in Synchro, you are giving up the ability to move across the entire cassette range. But that's the whole point of Synchro, to have the system shift up or down to the next useful gear.

FWIW, I figured out that I was rarely using the 11,12& 13 cogs in my big ring. I might and infrequently drop to the 50/11 to gain some speed on a downhill, but rarely. I never use the 34/11 either. I then did a gear chart and discovered if I kept my 34/50 crank and went to a 14-28 cassette, I had use of the 15-21 cogs on my big ring for the bulk of my riding, plus gained some hill gears on the 34/23-25-28. I found a 14-28 on sale and use that.