"Vérifie avec ta règle si ça mesure six millimètres."

Translation: Check with your ruler if that measures six millimeters.

[BobWells5]

The British spelling of 'millimetre' should be accepted. A 'meter' is a measuring device not a measure.

[k.odam]

Duo's use of English spelling is a bit inconsistent. «ça» can be "it" or "that", but if you spell out "millimetres" then "it" is rejected, and you have to use "that".

[LaBiciEsMia]

"check with your ruler if it measures 6 millimetres" should be accepted

[George583678]

I'd use calipers. Maybe for six centimeters.