Mach or Knots

I was performing flight RJTT to RJNK today when I received a mach violation. I couldn’t set the unit in the speed AP to Mach as my flight plan speed was in Mach. Therefore I used google to convert and the calculations gave me a violation. My question is, how do I set the unit to Mach or must I cross a certain altitude? Like FL300?

Once you cross FL 280 it turns in to Mach SPD on A/P.

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You must cross 28,000 ft for the speed to change to Mach.

EDIT: @Ark_of_Ahmed.IF too fast for me

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Ahhhh I was at FL270 thanks!

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Thanks, guys! :)