What I did to get rid of that same error was to add one line to /etc/default/elasticsearch
From the config file
# The maximum number of bytes of memory that may be locked into RAM
# Set to "unlimited" if you use the 'bootstrap.memory_lock: true' option
# in elasticsearch.yml.
# When using Systemd, the LimitMEMLOCK property must be set
# in /usr/lib/systemd/system/elasticsearch.service
Hope that helps.