I don't have experience as a DBA. Anyway, I have a problem that the backup drive is almost full (the free space just 7 GB) and the recommendation is to delete old backups manually and not using maintenance cleanup task.
The key words here are "I don't have experience as a DBA". That and the fact that you've got this particular problem likely also means that you don't actually have a Backup Plan or, more importantly, a Restore Plan if all hell breaks loose.
I strongly recommend that we work on that to start with.