Well its not really an error, its telling you that you don't have a definition for that lookup defined so its implicitly assuming you mean the one that its spotted in a lookup directory with the same name.
You can define the lookup in your transforms.conf to remove this notice, e.g.
[lookup_test]
filename = lookup_test.csv
Then you can just use | lookup lookup_test ....
Also, you don't need to do locno as locno, you only need AS if you need to rename the field to match the field names in the lookup, in this case its the same as your Splunk field so you don't need to include AS at all.
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