since you are doing an inputlookup, you need a | character as the first part of that subsearch. If I were you though since you are using a lookup table to join the data, why not just use he "lookup" command itself? You could just do:
index="abc" source_type="xyz" | lookup abcdef fieldX as fieldA
Looks fine to me, other than misspelling sourcetype.
Try this and see what happens -
| join type=left fieldA [ inputlookup abcdef.csv | rename fieldX as fieldA ]
| table fieldA ... some sample fields from main search... the fields from the lookup table...
| fillnull value="((none))"
Then the records that have all the lookup values set to ((none)) are the ones where no matching value for fieldA was found in fieldX.