If you were on tests-passed then a fresh 2.8 beta 9 install work won’t work for you, which is why you can’t use v2.9.0.beta9 as suggested by someone else above. As the name indicates, tests-passed is the latest version of merged changes which has passed the requisite suite of tests.
You’re going to need to pin the exact version of Discourse in your app.yml by inspecting your backup file.
If the plugins you use have been updated since the build in your backup then you may also need to pin those versions in the plugin section of your app.yml.