Have you tried resetting the admin password?
https://docs.splunk.com/Documentation/Splunk/7.3.1/Security/Secureyouradminaccount
You must have the ability to write to the underlying password file ($SPLUNK_HOME/etc/passwd).
splunk cmd splunkd rest --noauth POST /services/admin/users/admin "password=<your password>"
You must restart Splunk Enterprise after making this change.
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