Ok, there are a number of problems. The first problem is that no matter what else is going on, THIS is not how things should fail. There is no clue what is going on here.

The next thing that is happening is that the OP appear to be new to Malloy, welcome! ... and not entirely clear on what a source and a view and a query are, as Malloy concepts.

Running run: tests -> { select: * } you will see an output with three columns, the count, the nested list of carriers, and test_dim. nest_thing in the run query is a result of running nest thing in source query, it is an array of records. It is NOT reference to the nest view, it is reference to the result. The run query, expanded out a little, is this ...

run: tests -> { nest: cause_error is ( group_by: ANY_COLUMN_WHICH_IS_AN_ARRAY }};

The bug is we can't have a group_by on an array column value, in a nest statement, which I am working on fixing.

I think OP probably means something like this ...

// add a measure, a view, and a dimension to flights
source: tests2 is duckdb.table('../data/flights.parquet') extend {
    measure: tests_count is count()
    view: nest_view is {
        group_by: carrier
     }   
    dimension: test_dim is @2024
}

// run a query using the measure and the view
run: tests2 -> {
  aggregate: tests_count
  nest: nest_view
}

Feel free to bug us on slack, we'd be happy to walk with you on the learning Malloy journey.

Here's a pared down example

source: one_rec is duckdb.sql("SELECT {num:1} as num_rec")
run: one_rec -> { nest: next_rec is {group_by: num_rec }}

source: one_num is duckdb.sql("SELECT 1 as num")
run: one_num -> { nest: next_rec is {group_by: num }}

The first example gets the same "Parser Error: syntax error at or near "as" ad the OP report. The second one works.

The problem is the group_set is not generarted properly because the num_rec is a FieldInstanceResult and so the stage zero field generation skips it.

source: tests is duckdb.table('../data/flights.parquet') -> {
    aggregate: tests_count is count()
    nest: nest_view is {
        group_by: carrier
    }
} extend {
    dimension: test_dim is @2024
}

This feels wrong.

run: tests -> {
    nest: nest_view
}

Wouldn't you run:

run: tests -> {
   group_by: nest_view. -- or 'select: nest_view'
}