setdiff function requires x,y inputs of the same class. If x is a dataframe and y is a vector how do I use this function?

The data just has to be representative to the extent to illustrate the problem. It doesn't have to be all the data, your data or even fake data just so long as the code produces the same problem for which help is needed. Ideally, a reprex (see the FAQ is cut-and-paste into a session and allows everyone to get to the same sticking point.

The simplest way to include representative data is to use

dput(my_data_object)

Here's an example with mtcars

# with dput(mtcars)
the_data <- structure(list(
  mpg = c(
    21, 21, 22.8, 21.4, 18.7, 18.1, 14.3,
    24.4, 22.8, 19.2, 17.8, 16.4, 17.3, 15.2, 10.4, 10.4, 14.7, 32.4,
    30.4, 33.9, 21.5, 15.5, 15.2, 13.3, 19.2, 27.3, 26, 30.4, 15.8,
    19.7, 15, 21.4
  ), cyl = c(
    6, 6, 4, 6, 8, 6, 8, 4, 4, 6, 6, 8,
    8, 8, 8, 8, 8, 4, 4, 4, 4, 8, 8, 8, 8, 4, 4, 4, 8, 6, 8, 4
  ),
  disp = c(
    160, 160, 108, 258, 360, 225, 360, 146.7, 140.8,
    167.6, 167.6, 275.8, 275.8, 275.8, 472, 460, 440, 78.7, 75.7,
    71.1, 120.1, 318, 304, 350, 400, 79, 120.3, 95.1, 351, 145,
    301, 121
  ), hp = c(
    110, 110, 93, 110, 175, 105, 245, 62, 95,
    123, 123, 180, 180, 180, 205, 215, 230, 66, 52, 65, 97, 150,
    150, 245, 175, 66, 91, 113, 264, 175, 335, 109
  ), drat = c(
    3.9,
    3.9, 3.85, 3.08, 3.15, 2.76, 3.21, 3.69, 3.92, 3.92, 3.92,
    3.07, 3.07, 3.07, 2.93, 3, 3.23, 4.08, 4.93, 4.22, 3.7, 2.76,
    3.15, 3.73, 3.08, 4.08, 4.43, 3.77, 4.22, 3.62, 3.54, 4.11
  ), wt = c(
    2.62, 2.875, 2.32, 3.215, 3.44, 3.46, 3.57, 3.19,
    3.15, 3.44, 3.44, 4.07, 3.73, 3.78, 5.25, 5.424, 5.345, 2.2,
    1.615, 1.835, 2.465, 3.52, 3.435, 3.84, 3.845, 1.935, 2.14,
    1.513, 3.17, 2.77, 3.57, 2.78
  ), qsec = c(
    16.46, 17.02, 18.61,
    19.44, 17.02, 20.22, 15.84, 20, 22.9, 18.3, 18.9, 17.4, 17.6,
    18, 17.98, 17.82, 17.42, 19.47, 18.52, 19.9, 20.01, 16.87,
    17.3, 15.41, 17.05, 18.9, 16.7, 16.9, 14.5, 15.5, 14.6, 18.6
  ), vs = c(
    0, 0, 1, 1, 0, 1, 0, 1, 1, 1, 1, 0, 0, 0, 0, 0,
    0, 1, 1, 1, 1, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0, 1
  ), am = c(
    1,
    1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1,
    0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1
  ), gear = c(
    4, 4, 4, 3,
    3, 3, 3, 4, 4, 4, 4, 3, 3, 3, 3, 3, 3, 4, 4, 4, 3, 3, 3,
    3, 3, 4, 5, 5, 5, 5, 5, 4
  ), carb = c(
    4, 4, 1, 1, 2, 1, 4,
    2, 2, 4, 4, 3, 3, 3, 4, 4, 4, 1, 2, 1, 1, 2, 2, 4, 2, 1,
    2, 2, 4, 6, 8, 2
  )
), row.names = c(
  "Mazda RX4", "Mazda RX4 Wag",
  "Datsun 710", "Hornet 4 Drive", "Hornet Sportabout", "Valiant",
  "Duster 360", "Merc 240D", "Merc 230", "Merc 280", "Merc 280C",
  "Merc 450SE", "Merc 450SL", "Merc 450SLC", "Cadillac Fleetwood",
  "Lincoln Continental", "Chrysler Imperial", "Fiat 128", "Honda Civic",
  "Toyota Corolla", "Toyota Corona", "Dodge Challenger", "AMC Javelin",
  "Camaro Z28", "Pontiac Firebird", "Fiat X1-9", "Porsche 914-2",
  "Lotus Europa", "Ford Pantera L", "Ferrari Dino", "Maserati Bora",
  "Volvo 142E"
), class = "data.frame")