Which CI are you using? We can only automatically detect a PR analysis on some CIs, not all. If the CI is not supported you have to set the sonar.pullrequest.key parameter, see the documentation for more information.
That is not one of the CI services we support. I suggest using the $BITRISE_PULL_REQUEST environment variable (documentation) exposed to set the sonar.pullrequest.key parameter when the environment variable is available.