Ignore all slicers except for one

As previously mentioned, this will return a table:

FILTER ( ALL ( 'CY'[Slicer] ), 'CY'[Slicer] = SELECTEDVALUE ( 'CY'[Slicer] ) )

If you put this in a separate measure, you'll get something like:

The expression refers to multiple columns. Multiple columns cannot be converted to a scalar value. This alone will cause an error.  SELECTEDVALUE ( 'CY'[Slicer] ) should have sufficed but even so, this will cause an error. As mentioned, the  third argument in DIVIDE must be a constant.  If you dissect the DIVIDE within your IF conditions, the lines in below below are still not constants -  they're variables that are dependent on the value of the current filter contexts or other calculations - they should be a plain number or a text string.

=
IF (
DIVIDE (
CALCULATE (
( Output[Output TY] ),
ALLSELECTED ( 'Output' ),
'Output'[P&L] IN { "Gross Profit" }
),
CALCULATE (
( 'Output'[Output TY] ),
ALLSELECTED ( 'Output' ),
'Output'[P&L] IN { "Net Sales" }
),
FILTER ( ALL ( 'CY'[Slicer] ), 'CY'[Slicer] = SELECTEDVALUE ( 'CY'[Slicer] ) )
) = 0
|| ISBLANK (
DIVIDE (
CALCULATE (
( Output[Output TY] ),
ALLSELECTED ( 'Output' ),
'Output'[P&L] IN { "Gross Profit" }
),
CALCULATE (
( 'Output'[Output TY] ),
ALLSELECTED ( 'Output' ),
'Output'[P&L] IN { "Net Sales" }
),
FILTER ( ALL ( 'CY'[Slicer] ), 'CY'[Slicer] = SELECTEDVALUE ( 'CY'[Slicer] ) )
)
),
SELECTEDVALUE ( 'CY'[Slicer] ),
DIVIDE (
CALCULATE (
( Output[Output TY] ),
ALLSELECTED ( 'Output' ),
'Output'[P&L] IN { "Gross Profit" }
),
CALCULATE (
( 'Output'[Output TY] ),
ALLSELECTED ( 'Output' ),
'Output'[P&L] IN { "Net Sales" }
),
FILTER ( ALL ( 'CY'[Slicer] ), 'CY'[Slicer] = SELECTEDVALUE ( 'CY'[Slicer] ) )
)
)

Instead of writing this very log formula, I would have used variables instead or write those within calculate in a separate measure.  I don't also see why use an IN operator when there's only one value in the list after it.  { "Gross Profit" } - there is only "Gross Profit" inside the curly brackets, nothing else.

=
VAR _GP =
    CALCULATE (
        ( Output[Output TY] ),
        ALLSELECTED ( 'Output' ),
        'Output'[P&L] = "Gross Profit"
    )
VAR _NS =
    CALCULATE (
        ( 'Output'[Output TY] ),
        ALLSELECTED ( 'Output' ),
        'Output'[P&L] = "Net Sales"
    )
RETURN
    IF (
        ISBLANK ( DIVIDE ( _GP, _NP ) )
            || DIVIDE ( _GP, _NP ) = 0,
        SELECTEDVALUE ( 'CY'[Slicer] ),
        DIVIDE ( _GP, _NP )
    )

or separate GROSS PROFIT and NET SALES mesures to be able to reuse them later

Gross Profit =
CALCULATE (
    ( Output[Output TY] ),
    ALLSELECTED ( 'Output' ),
    'Output'[P&L] = "Gross Profit"
)

Net Sales =
CALCULATE (
    ( Output[Output TY] ),
    ALLSELECTED ( 'Output' ),
    'Output'[P&L] = "Net Sales"
)

DIVIDE Measure =
VAR _DIV =
    DIVIDE ( [Gross Profit], [Net Sales] )
RETURN
    IF ( _DIV = 0 || ISBLANK ( _DIV ), SELECTEDVALUE ( 'CY'[Slicer] ), _DIV )