upload and sync folders with rsync

I would like to sync a folder i have on a linux/debian machine with a folder used for my webpage. rsync would be most flexible but I get error messages:

• rsync -v aaa.txt user@domain.tld:/test1
• rsync: connection unexpectedly closed (0 bytes received so far) [sender]
• rsync error: unexplained error (code 255) at io.c(235) [sender=3.1.2]

what do I wrong? to make exchanges more flexible, how exchange ssh key?