Cricket Wireless has not yet updated the Samsung Galaxy S6 (SM-G920AZ) to Marshmallow 6.0.1 yet.
AT&T has updated there Samsung Galaxy S6.(SM-G920A) Even UNLOCKED cricket wireless versions of the Samsung Galaxy S6 do not yet have access to marshmallow 6.0.1.
That being said, I have contacted Cricket technical support and they say the hold up is Samsung, however Samsung said it released all Marshmallow 6.0.1 files to the companies. So its either a Cricket or AT&T issue that is holding up the update. My feeling is it is probably AT&T delaying the update.
But who really knows. Hopefully Cricket will get the update out by the end of the month.