The oneworld RTW booking tool is almost a complete waste of time, far more often than not leading to utter frustration.

The 'Your itinerary exceeds to maximum 34,000 miles allowed' error is generic and seems to be the default when there's any kind of issue, 34K or not. For example, it will display if you've got 17 segments instead of 16 (and even when you've selected 16 segments, the stupid tool will often autoselect an indirect route, adding another sector without you even knowing, such is uselessness).

In your case, I suggest selecting the flights you want. This will ensure you're not getting stuck on the above issue, plus other ones like, for example, you entering SFO-JFK-LHR-LYS, but the tool deciding what you really want it SFO-JFK/LGA-LGW/LHR-LYS (thereby, again, adding extra sectors). Check also that you're not inadvertently creating stopovers where you intend transits, due to lack of availability in the required fare class. If this step fails and you still get the error, try removing a couple of sectors at a time (e.g. enter SFO-LHR-LYS) and go from there in a process of elimination.

And if all that fails, find a travel agent or phone the AA RTW desk whose number I don't have to hand but you'll likely find it in this thread or this one.