Yukon XL 2015 won't start occasionally when brake won't depress

If I had the answer I would not be asking the above question.

You have to depress the brake peddle fully to engage the starter. If you don't press the peddle, you get accessory mode. If brake peddle is pressed but won't go down all the way, the computer reads that as a defect that needs to get repaired, and that's why the ignition light flashes. It's difficult for a mechanic to diagnose intermittent problems when they don't happen in the shop, even when trouble codes are read. Your problem is likely a failing brake master cylinder or an intermittent vacuum loss..... When this fails completely, it will be easier to diagnose.