Apps themselves don't have "access to data". What regulates access or not are the roles of the user running a search, that may not have access to specific indexes.
Can you open of the panels in the dashboard, and extract the search that is underneath it and run it yourself?
If you can see the results, than the roles hypothesis gains momentum. Let me know
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The answer looks like this (inside a docker container running the free enterprise version)
sudo -u splunk /opt/splunk/bin/splunk install app ./splunk-machine-learning-toolkit_500.tgz -update 1
See
$ splunk help install
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