Rsync error code 23

I am trying to back up a server into a flashdrive with rsync:

rsync -rv --delete /home/computername/filename /dev/disk/by-label/Flashdrivename

It fails, with:

sending incremental file list
rsync: delete_file: unlink(Flashdrivename) failed: Permission denied (13)
could not make way for new regular file: filename

sent 53 bytes received 138 bytes 382.00 bytes/sec
total size is 0 speedup is 0.00
rsync error: some files/attrs were not transferred (see previous errors) (code 23) at main.c(1196) [sender=3.1.2]

if someone knows what's the problem i would be very grateful if you told me.

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