Paul,

You can't use a SetValue action to assign a value to an AutoNumber.  In
fact, normally you do not assign the value to an Autonumber, as this is
done automatically by Access when a new record is started.  As far as I
know, the only way you can control the value of an autonumber is to use
an Append Query to add a new record with the number you want (assuming
there is not already a record with this number assigned.