If you follow the steps, but you still see this:
go to https://sentry.io/settings/$organization/projects/$project/plugins/trello/ and click the “Enable plugin” button in the top right.
There are also less useful but official instructions here: https://github.com/getsentry/sentry/blob/a0b764384b26b2ac008e31a837b37921783b1033/src/sentry_plugins/trello/Trello_Instructions.md