I have the same issue with my github page. I'm using gohugo with this theme https://themes.gohugo.io/themes/hugo-theme-tailwind/
And this is my configuration

  [params.giscus]
    enabled = true
    repo = "hntan/hntan.github.io"
    repoID = "167350460"
    category = "General"
    categoryID = "41801912"
    mapping = "pathname"
    strict = "0"
    reactionsEnabled = "1"
    emitMetadata = "0"
    inputPosition = "bottom"
    lang = "en"

@hntan I see your blog has the giscus comments functional now. Can you share how you got it working? It seems that multiple users have the same issue.

@ElastiCourse , Those IDs are actually generated by Giscus, not GitHub. To retrieve them, visit https://giscus.app/ and enter your repository and category information. Then, simply copy the generated ID.

@hntan I seem to have found the issue. The https://giscus.app/ was giving incorrect repo ID (missing one character at the end). For some reason when I entered the repo name again today, it gave the correct FULL repo ID.

For anyone having issues with giscus 404/400 errors, verify the repo ID configured is correct. Here is an example curl command that you can run without login (change owner and repo name & make sure jq is installed)

curl -s "https://api.github.com/repos/ElastiCourse/ElastiCourse-community" | jq '.node_id'

Output:

"R_kgDOMiAWtw"

this seemed to be the issue in my case too.
Thanks!