Hi Scott. Sorry that you ran into some issues. I think the docs are missing a step here for the Sandbox. Can you try installing the standalone configuration RPM for Accumulo as well?
# yum install accumulo-conf-standalone
The standalone configuration RPM should install some sane defaults which work on the Sandbox. The Accumulo RPM just installs the software, not a set of configuration files since the configuration is highly dependent on your other configuration, such as HDFS Namenode and ZooKeeper quorum servers.
For some extra context, ACCUMULO_HOME should be symlinked to by /usr/hdp/current/accumulo-client with ACCUMULO_CONF_DIR being /etc/accumulo/conf.