What version of Splunk are you using? If you haven't done so already try updating to 6.4 as this process seems to work more smoothly in the latest version. You may initially see a message in the troubleshooting dashboard that the certificate is invalid, but restarting Splunk should take care of that.
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You're missing the opening ". It should start with
"keyCredentials": [{"customKeyIdentifier" ...
and end with
... ", "keyId": "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"}]
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When copying the auto generated key credentials, make sure to replace everything between the two commas surrounding the key credentials section in the manifest. If still in doubt use a JSON editor to check your JSON.
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