Converting 1x1 array to number

_Elwetana 

For the single row range INDEX returns single value, not an array. You may check with

= TYPE( INDEX(22:22,,2) )

which gives 1.

However, for multiple rows, e.g.

= TYPE( INDEX(22:23,,2) )

 it gives 64 which means an array.

In general, with your formula perhaps you mean something like

=LET(
    src_range, A2:AA21,
    cur_row, A22:B22,
    myFunc, LAMBDA(range,parRow,
        LET(
            start, INDEX(parRow, , 1),
            end, INDEX(parRow, , 2),
            IF( start = 0, "start column shall not be blank",
            IF( end = 0, "end column shall not be blank",
            IF( end < start, "start column shall be more than end one",
                CHOOSECOLS( src_range, SEQUENCE(, end - start + 1, start ) )
            )))
        )
    ),
    myFunc(src_range, cur_row )
)