Garbage collection because no space left, can I deleted them manually? Or any other thing I can do?
You don't know which chunk belongs to what backup snapshot. So by randomly deleting chunks you will probably corrupt most of your backups snapshots.
Then there aren't many options if you can't run a GC.
If you don't care about losing all your backups you could just delete the datastore folder and start from scratch with a new datastore.
If you care about your backups you can only:
1.) buy more disks and add them to the pool so you get more space
2.) mount a bigger filesystem (NFS/SMB share or USB disk), copy the whole datastore to it, point PBS to the new datastore path and then run a GC
3.) mount some additional storage, move some of the chunks to the new storage and then symlink them to your datastore, run the GC and later move them back
So all options require having some additional storage.
For the future, you should really set up some quotas. A ZFS pool shouldn't be filled too much anyway as it will become slow when full. Usually recommended is not to fill it more than 80-90%. So I would set a 90% quota so you can't fill it more than 90% by accident. If you then will it to 90% where everything will stop working you could increase the quota again to something like 95% so you get some free space so you could at least run the GC to free up some space.