In order to work, HACS needs to retrieve information about repositories using Github's API.
Because of the rate limits set by Github, HACS needs to be authenticated by a Personal Access Token, that you can generate using the following steps.
You are probably looking at this in a browser, so we can probably check this off.
And then "Personal access tokens." or click here: https://github.com/settings/tokens
Click the "Generate new token" button.
If you are asked to login, do so.
First give it a logical name so that you can recognize it.
Then click the "Generate token" button at the bottom.
You do not need to check any of the boxes.
Now you see the generated token, this will be the only time you see it, make sure that you copy it manually or by clicking the clipboard icon.