Hi @dwood1,
The error indicates that you used an invalid method, particularly when calling api.redirect.sendUserTo().
I have checked our Actions Triggers: post-change-password - API Object documentation and confirmed that there isn’t a redirect method available.
This is only available in the onExecutePostChallenge trigger.
If you would like the user to be redirected to a specific URL after they have reset their password, you can set that value in your Change Password (link) email template under the "Redirect To’ field.
For more details, I recommend referring to our Password Change Redirect Behavior knowledge solution.
Cheers,
Rueben