Assuming that you’ve removed them from the relevant processes, the issue here is that they’re still referenced by a release. We did put an enhancement in Octopus version 2019.7.2 that does allow these to be removed while they’re in this situation. You can find details of the issue/enhancement here: https://github.com/OctopusDeploy/Issues/issues/5681
Sadly there isn’t a workaround for prior releases, so it will be something that will be resolved in your next upgrade.
Please let me know if you have any further questions!