If you are using the "Technical Addon for the Metricator application for Nmon" to monitor your indexers in an index cluster, the TA does not work after updating to Splunk 9.0. This is due to the name change in the Splunk path.
The fix is to update all the scripts in the TAs bin folder.
# Discover TA-metricator-for-nmon path
"$SPLUNK_HOME/etc/slave-apps/TA-metricator-for-nmon" TO "$SPLUNK_HOME/etc/peer-apps/TA-metricator-for-nmon"