Add 'n' number of days(excluding weekends and holidays) to a given date.

Alright, I found workday() function will help in this regard.

That should help you - my additional thought was, if you're storing your holidays in your database (as opposed to going through the fairly cumbersome and not-very-flexible process of defining them manually within your Appian environment), a brute-force approach would probably be feasible too - i.e. loop over a date range (like with a!forEach) and check each date for its "day of week" as well as whether it's in your holidays DB.

Yea, I have this setup as a recursive function in some areas where additional processing is required.  Generally, fn!workday() should cover it, but if we need to expand the logic, a starting example is:

rule!chris_test_add_days():

a!localVariables(
  local!dateStart: today(), /* replace with rule input if you do not always want to add from today */
  local!addDays: 20,
  local!holidays: a!queryEntity(
    entity: cons!CHRIS_TEST_DS_HOLIDAYS,
    fetchTotalCount: false,
    query: a!query(
      /* make sure you include enough range - get the same # of future holidays as days adding */
      pagingInfo: a!pagingInfo(1,local!addDays,a!sortInfo(field: "date",ascending: true)),
      filter: a!queryFilter(
        field: "date",
        operator: ">",
        value: local!dateStart
      )
    )
  ).data,
  
  rule!chris_test_add_days_recur(
    date: local!dateStart,
    addDays: local!addDays,
    daysAdded: 0,
    holidays: property(local!holidays,"date",{})
  )
)

rule!chris_test_add_days_recur(date (Date), addDays (Integer), daysAdded (Integer), holidays (List of Date)):

if(
  ri!daysAdded=ri!addDays,
  ri!date,
  a!localVariables(
    local!nextDay: todate(caladddays(ri!date,1)),
    rule!chris_test_add_days_recur(
      date: local!nextDay,
      addDays: ri!addDays,
      daysAdded: ri!daysAdded+if(
        contains(
          ri!holidays,
          local!nextDay
        ),
        0,
        1
      )
    )
  )
)

As always with recursive functions, make sure you save your edits prior to testing!