TNS-12518: TNS:listener could not hand off client connection
 TNS-12536: TNS:operation would block
  TNS-12560: TNS:protocol adapter error
   TNS-00506: Operation would block
    Linux Error: 11: Resource temporarily unavailable

It turns out the Linux Error: 11: Resource temporarily unavailable was caused due my nproc limit being set to low.
Within the /etc/security/limits.conf file I had my oracle user set at 131072 nproc value
(oracle’s best practice from their oracle-validated rpm), however my grid user nproc value was set at 2047!
I changed the nproc value of my grid user to 131072 and now the issue no longer occurs.
nproc is the max number of processes that can be run by a user.