When attempting to reactivate a user through the API with an API Token, using POST /api/v1/users/${userId}/lifecycle/reactivate, the response returns an Invalid Session error from Okta:
 

{    "errorCode": "E0000005",    "errorSummary": "Invalid session",    "errorLink": "E0000005",    "errorId": "{ID}",    "errorCauses": []}
 

• Okta API
• Okta User Reactivation
• API token
• User Lifecycle Management

Invalid Authorization header in the request or an invalid API token.

Try the following troubleshooting steps:

1. Verify that the API key is valid.
2. Create a new API token from the Okta UI and use the new API token in Postman. 
3. Update Postman.

If none work, consider opening a case with Okta Support for further troubleshooting.