Hello Garuda users.
After a while my snapshots got older and system started to give a notification about old "snaphot delete" warnings. Another user also opened a topic about it. Some members adviced to use Garuda Maintenance and enable/disable notification for old snapshot.
But I would like to remove old snapshot backup to free disk and neither command line traditional deleting process nor BTRFS Assistant did not work due to this error:
Cannot delete snapshot <num> since it is the next to be mounted snapshot.
I'd like to remove writable copy of 102# that 115# and was getting that message. So I tried to rollback with BTRFS Assistant what could not go right, I'm awkward to use this tool, LOL 
And then used Snapper command line in terminal to rollback a healthy snapshot.
╰─λ sudo snapper --ambit classic rollback 481
Reboot and nice clean backup. cli always works with snapper. And rebooted and then removed old snapshot with
╰─λ sudo snapper delete 115
Tadaaa, it gone and saved disk space. Hope this short topic can help other users to rid off old backup/snapshots.
*I'm novice user of BTRFS and snapshots, still cannot get it completely, awkwardness 