On Airbus aircraft¹, the "RETARD" callout occurs just before touchdown and acts as a reminder² for the pilot to put the thrust levers to idle. Regardless of whether the AP/AT is engaged or not - regardless of whether the thrust levers have already been "retarded" or not, this callout will reiterate from the time it starts until main-gear touchdown³.
If the A/Thr is engaged (like during autoland), it will automatically command idle power at the appropriate time during flare, but it is still necessary to manually retard the thrust levers to idle in order to be able to engage reverse thrust after touchdown⁴.
¹All Airbuses other than A-300, A-310 and A-220.
²This is a reminder - this is not a command. Ultimately, the pilot must decide exactly when it would be appropriate to retard the thrust levers based on the situation.
³Sometimes this callout will begin, but then shortly will be replaced with radio altimeter callouts.
⁴And even if reverse thrust need not be used, retarding the thrust levers is still necessary for the automatic deployment of ground spoilers and for autobrake activation.