is there a diagnostic fir this?

 

If your vehicle has a switch located on the brake master cyl , check that sw. When brakes is applied it cancels cruise. If sw is stuck it will not allow cruise to set.

 

Like hanky mentioned, could be the canceling switch fail or it’s connector for corrosion etc.
There’s not much shop info on system diagnosis available, but there is a schematic available to check steering wheel switches, Clock spring, harness on down to the pressures switches as well as the servo unit. With a schematic, you could check for input/output and prove out circuit functions as part of a diagnosis..