Destination SDK Failing because of invalid Auth token(Authencation Failed)

Two good points you raised.

Sending segment name instead of segment ID

Yes you can. Update your server template request body with this:

"requestBody": {
  "templatingStrategy": "PEBBLE_V1",
  "value": "{\n  \"items\": [\n    {% for profile in input.profiles %}\n    {\n      \"firstname\": \"{{ profile.attributes.firstname }}\",\n      \"lastname\": \"{{ profile.attributes.lastname }}\",\n      \"emailaddress\": \"{{ profile.attributes.emailaddress }}\",\n      \"segmentName\": \"{{ destination.namespaceSegmentNames[input.aggregationKey.segmentNamespace][input.aggregationKey.segmentId] }}\",\n      \"segmentStatus\": \"{{ input.aggregationKey.segmentStatus }}\"\n    }{% if not loop.last %},{% endif %}\n    {% endfor %}\n  ]\n}"
}

The SDK automatically resolves the actual segment name from AEP at runtime — so whatever you've named your segment in AEP, that's exactly what lands in SFMC. No hardcoding needed. segmentStatus will tell you whether the profile entered (realized) or left (exited) the segment, which is useful for suppression logic in SFMC.

Before testing — make sure your SFMC Data Extension has these two columns added:

• segmentName (Text)
• segmentStatus (Text)

If those columns don't exist in the DE, the values will be silently dropped and you won't see any error.

On your schemaConfig.profileFields question — you're right, you don't need to add anything there

profileFields is only for profile attributes that need to show up in the AEP mapping UI — like firstname, lastname, emailaddress where you're manually mapping an AEP attribute to a destination field.

segmentName and segmentStatus are resolved automatically by the SDK from the aggregation context at export time. They don't come from the profile, they come from the segment itself — so there's nothing to map and nothing to add to profileFields. Your aggregation config already has includeSegmentId: true and includeSegmentStatus: true which is all that's needed for these variables to work.

Just update the server template and add the two columns in SFMC. 

Let me know how it goes.