In addition to @Hooverdan’s recommendation, I would also like to take the opportunity to guide you towards the new Rockstor release announced by @phillxnet below:
You will there see that we started switching from AFP/netatalk to using the Samba protocol, which is now the supported way to use Time Machine. For instructions, I would guide you towards the post below, but make sure to disable the AFP service first, especially given it is the likely culprit to explain the error you reported above.