Wireless Access

We have two 7205 controllers (primary/secondary) with OS 8.11.1.2  and 168 AP-615's.  Does anyone have this similar environment?

I am struggling to get the 6Ghz band to work in our environment. 

I have created an SSID with WPA3 security and turned on the 6ghz band. 

I have been able to broadcast it ... but the few 6Ghz compatible devices wont connect under 6Ghz band,

Can 2.4 , 5, and 6 all coexist?

How do you have you have your environment setup?

Are you seeing the 615 broadcasting the WLAN on 6 GHz?  Since the 615 is a flex-band AP you have to either manually configure 2.4/6 or 5/6 operation or AirMatch has to create a solution that moves the AP to operating in 6 GHz.

You'll need to make sure that your 6 GHz RF coverage is good enough to support roaming throughout your environment. Current client behavior seems to prefer 6 GHz when found on initial connection and the 6 GHz signal is better than 5 GHz.  Problems seem to arise when your 6 GHz has a gap in coverage but other bands are available, if the client roams through that spot they have a tendency to stick with whatever band they move to. 

As chulcher said, please verify if 6GHz SSIDs are broadcasted by those APs

show ap bss-table will shows you that - https://www.arubanetworks.com/techdocs/CLI-Bank/Content/aos8/sh-ap-bss-table.htm

You can restrict the list by 6GHz only with

show ap bss-table | include "6GHz/"

How did you setup your "AP System" Profile? To allow the usage of 6GHz on 615 AP you have to dedicate choose the operating mode:

As far as I understand you don´t have a Mobility Conductor in place, so the Automatic option does not work for you. With Automatic you have to have a Mobility Conductor which provides AirMatch Service and calculates the best Radio Usage.
See - https://www.arubanetworks.com/techdocs/ArubaOS_8.11.0_Web_Help/Content/arubaos-solutions/access-points/conf-ap-syst-prof.htm

Stand-alone Controllers is you operating mode without a Mobility Conductor.