This is the same error that prevented me from installing 16.6.1...I was able to install and run the drivers even through successive reboots by simply copying atdcm64a.sys from c:\AMD\ the 16.5.3 folder\bin64 to the same folder in c:\AMD\ 16.6.2driver folder\bin64\ (I renamed the 16.6.2 version of atdcm64a.sys to atdcm64a.sys1662 .) Then I ran setup.exe from the c:\AMD\16.6.2 driver folder\ and the 16.6.2 drivers installed without a hitch. [These instructions assume the 16.6.2 driver files have already been decompressed into c:\AMD, of course.]
It's still the certificate problem.
MSI 970A Gaming mboard UEFI version 22.4 (M4)/Corsair CX-750M PSU (62a x1 12v rail)/AMD FX-8320E @ 3.2 GHz/
RAM 8GB 2x4GB, 2133MHz interleaved, dual channel, 11-12-12-30-48-1T/LG MultiDrive DVD/Boot:Samsung EVO 850 256GB (UEFI boot partition)/
2x 1TB WD Blue 7200 rpm/RAID 0: ST3350041 8AS SATA (x2)/MSI AMD R9 380 4GB/Realtek 1150 sound
If anyone has a more permanent solution I'm all ears...;)