What I did to get rid of that same error was to add one line to /etc/default/elasticsearch

    cat /etc/default/elasticsearch
    MAX_LOCKED_MEMORY=unlimited

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
    #MAX_LOCKED_MEMORY=unlimited

Hope that helps.
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