Do you have the same problem enrolling other Samsung device models as AE devices onto the same server?  If so, maybe there is some firewall problem for your devices connecting to Samsung ELM server in your region. 

If not,  then the problem may be related to the A02 device firmware itself. Please check if there is any KNOX libraries included in the firmware of your Samsung A02 devices.   As mentioned in some of my other earlier posts,  I heard from some Samsung guys that some low-end Samsung devices lack various KNOX libraries in their firmware, and such devices are not meant to be used by enterprise needing device managment with any MDM software.   This is unofficial, and you might need to verify this with Samsung support team.   I couldn't rule out possibility of any change(s) in the policies/ target market for such low-end devices.