The expression refers to multiple columns. Multiple columns cannot be converted to a scalar value

It's possible to have calculated tables inside a measure but if you want to return a table, then you need to use CALCULATETABLE rather than CALCULATE. However, since you are using _inadequate_security_ inside a SWITCH as if it is True/False, I suspect that you meant to have an argument in CALCULATE other than the filter table.

Did you intend to write something like this? (Note the line before FILTER.)

Inadequately secured loans =
VAR _inadequate_security =
    CALCULATE (
        ISEMPTY ( 'Table1' ),
        FILTER (
            'Table1',
            'Table1'[Collateral Amount] < 1.5 * 'Table1'[Loan Amount]
                && NOT ( 'Table1'[Product ID] IN { 2222, 8888 } )
        )
    )
VAR _result_ =
    SWITCH (
        TRUE (),
        _inadequate_security, "Inadeqaute Security",
        "Adequate Security"
    )
RETURN
    _result_