In order to delete a backup repository from the Veeam Backup & Replication UI, you must ensure that:
A. None of the jobs are running at the moment
B. At least one successful Configuration Backup has been made
C. None of the jobs, including the Configuration Backup job, are targeting it
D. All of the backup files are removed from this repository
Correct answer is C:
You cannot remove a backup repository that is used by any job. To remove such backup repository, you first need to delete a reference to this backup repository in the job settings.
https://helpcenter.veeam.com/docs/backup/vsphere/repo_delete.html?ver=110