I assume what you are asking is, why the indexers don't show up.
make sure your DMC instance is a search head of the indexer cluster. Go to Settings -> Indexer clustering -> enable clustering, then choose mode as "search head" and point to the cluster master, then restart.
go to Settings -> Distributed search -> Search peers, verify that the indexers are listed as peers on this page.
go to DMC setup page, now the indexers should be listed here, make sure they have the "indexer" server role. Then click "Apply Changes" button. Note clicking the "Apply Changes" button is important!
Now the overview page should show a new panel for the indexers and the indexing related dashboards should work.
NOTE: since your deployment is indexer cluster + search head cluster, please make sure the DMC is a search head in the indexer cluster while NOT being a search head cluster member. In another word, the DMC needs to be inside indexer cluster, and outside of search head cluster.
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