You are using the clients API so you need to add the manage-clients role in your user.
Edit:
To use the service account you need to authenticate with the client id and token.
curl --location --request POST 'http://localhost:8080/realms/HUB/protocol/openid-connect/token' \
--header 'Content-Type: application/x-www-form-urlencoded' \
--data-urlencode 'client_id=my-client' \
--data-urlencode 'client_secret=my-client-secret' \
--data-urlencode 'grant_type=client_credentials'
Then you can use this token to make your API calls.
Make sure that the service account has the required roles assigned.
This is my call to get the role users:
curl --location --request GET 'http://localhost:8080/admin/realms/HUB/clients/4eaeb1d8-3dd9-4e8d-a352-a71574dfdff1/roles/api-user/users' \
--header 'Authorization: Bearer eyJhbGciOiJSUzI1NiIsInR5cCIgOiAiSldUIiwia2lkIiA6ICJGUlJlRFdDV0FhY3QzOVRIYlFrOHpueEM2TS1YS2xrQ0Y3WWtOY2w4cmJzIn0.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.DdWOpa5wtIDYNy422AF5S6k-8DWnOB1PG-4olQ1DcV6TjLX-WZRiuoaHFGqCMqDkMhpVyu_xUv0QkmdgM73-rFvDw_DwagFYqA_OW4zMqk4Lp4nf46bKlAYBajZmFVwbEgfjoIgDup3oPjYi2BOKrinMtNzfrSpcKuIlqjc_aEH9dSApqYEcqVewk5AYfkIFO1B84utdt27XIvHFvg_JqS3sOMkS3qtUT1wHrLPQ8GfSIoBDvRVde6kYt3UvbRkV6yeqtoNwYj-Kwsr-FHgdCRfZbzTqYQGYb-9RQovfAciucg4uph7Zqm7xBeQXmTXVm1GaAL6kniU2KFSR7BwRWQ' \
--data-raw ''