Configuring Gitlab to Mirror Bitbucket Repo Getting Unauthorized

I am following this page in trying to setup a GitLab project to mirror a repository in Bitbucket and have the former do the CD/CI.

In doing so, I complete steps 1-3 and proceed to test the webhook by making pushing a change to the Bitbucket repo. Inspecting the requests of the webhook on the Bitbucket side, I see that it successfully triggered an event, but received a “401 Unauthorized” back from GitLab.

I am wondering what I can look into to troubleshoot this. The private token I created is of scope “api”, as per the documentation. There is not much else aside from the project number, which seems trivial and can be discarded.