I got the legend achievement for master chief collection about two months ago and have still not received the helioskrill armor for halo 5. There has been no error with the achievement itself and this website does confirm that I have completed all the missions, I also have the achievement unlocked on the xbox.
Have a try of the solutions mentioned in this thread.
I got the legend achievement but no helioskrill have you received yours yet?
I know its been a while but for anyone else that has the same problem I had. I just got on halo 5 around 5 or 6 months later and had helioskrill so maybe it’s just a long wait.