You need to make sure you have authorized Sentry for your Repo or Organization.
The owner of the repo needs to go to Account Settings > Authorized Applications and make sure that Sentry is an authorized application.
If you have a repo that is part of an organization on Github:
Visit https://github.com/settings/connections/applications/0dc8e2df8aa45477de5e and see if the organization has a red ‘x’. If so, you need to ‘Grant Access’ (which can be done by the owner of the organization).
Once you grant access you can go to Organization Settings > Third Party Access > Sentry to confirm the authorization.
If you are not the owner of the account you can follow these instructions.