I was experiencing this problem as well and it ended up being a permissions issue on the mongo folder itself. after reading through these posts as well as the referenced posts I did the following:
1) stopped splunk process
2) right clicked on mongo folder located in C:\Program Files\Splunk\var\lib\splunk\kvstore
3) selected security and altered the security permissions for local admin as well system, before exiting I selected the replace child object permissions entries with inheritable permissions entries from this object.
4) restarted splunk and my errors went away.
these steps cleared the error message Key value store must be enabled. Please enable it" and it enabled me to perform a KVStore migrate which i was not able to do before fixing this error. (KV Store initialization has failed.)
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