Hey,
most likely, you are experiencing this issue because your browser is configured to not accept untrusted connections. By default, Splunk ships with a self-signed default certificate, that is not trusted by any system and should be replaced (at least on production instances).
Therefore, if you replace that certificate with a valid one, or are able to tell your browser to ignore it, you should be able to proceed.
Hope that helps - if it does I'd be happy if you would upvote/accept this answer, so others could profit from it. 🙂
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