Error with GitHub’s Octokit while building GitHub App in GHES environment: https://api.github.com/app/installations/2/access_tokens: 404 — Integration not found

While building and testing a GitHub App through GitHub’s Octokit’s Ruby code, everything worked fine when I was working under a GHES environment. However, when I switched to the GHES environment.

2021–08–25 10:00:44 — Octokit::NotFound — POST https://api.github.com/app/installations/2/access_tokens: 404 — Integration not found // See: https://docs.github.com/rest