I think I've seen this issue before. The add-on is hosted on both https://prguidelines.atlassian.io and on https://prguidelines.useast.atlassian.io. But the oauth consumer for the add-on is based on https://prguidelines.atlassian.io (for its callback URL).
This means that if you installed the add-on via the https://prguidelines.useast.atlassian.io URL, there will be a mismatch between the consumer and the add-on base-url. I try to use the first URL everywhere I can, but clearly I've missed a spot. So the following steps should fix your issue:
Uninstall the add-on on that account.
Go to https://prguidelines.atlassian.io and click the button that starts the installation.
Make sure to target the same account as before.
To help me make sure that this doesn't happen in the future, can you tell me where you installed the add-on from?
Abhin ChhabraAccount Deactivated
So I think I know what happened. The URL set in the Bitbucket marketplace was actually the second one. That seems like a mistake and I've fixed it. You should be able to reinstall the add-on from the marketplace itself and your problem should go away.