Solved
Unified tags error while retrieving via API
Even though I'm using the right API key, I am still getting the API key is invalid.
https://experience.adobe.io/unifiedtags/tags?tagCategoryId=12345
(using the right tag category id)

Getting the 403 error
{
"error_code": "403003",
"message": "Api Key is invalid"
}
Documentation is not helpful in this case.

Any suggestion would help!