Ошибка 438 вба

First of all, you should know, that some of functions, used on the worksheet, have limitations. So my point is avoid of using them in VBA, if it is not necessary.
For example, function POWER() returns error on attempt to raise a zero to zero. An alternative is to use 0 ^ 0 combination, which is exactly doing the same, but looks more simply and operates without such error.
But also there is no embedded alternative in VBA to the FACT() function, so you can use it, or simply add your own function factor() — it’s uppon your choise.

If you just have started learning VBA, I would recomend you to use Option Explicit. It will help you to find out, which variables are not defined, and sometimes to avoid errors related to variable names missprint.

Here is your code, fixed and a little bit optimized:

Option Explicit' It is an option that turns on check for every used variable to be defined before execution. If this option is not defined, your code below will find undefined variables and define them when they are used. Good practice is to use this option, because it helps you, for example to prevent missprinting errors in variable names.

Sub Bezier()
    Dim C as Double , t As Double
    Dim k  As Long, n As Long, i As Long
    n = 3
    For i = 0 To 100
        t = i * 0.01
        Cells(i + 2, 6) = 0
        Cells(i + 2, 7) = 0
        For k = 0 To n
            C = (WorksheetFunction.Fact(n) / WorksheetFunction.Fact(k)) / WorksheetFunction.Fact(n - k)
            Cells(i + 2, 6) = Cells(i + 2, 6).Value + Cells(k + 2, 1).Value * C * (t ^ k) * ((1 - t) ^ (n - k))
            Cells(i + 2, 7) = Cells(i + 2, 7).Value + Cells(k + 2, 2).Value * C * (t ^ k) * ((1 - t) ^ (n - k))
        Next
    Next
End Sub

UPDATE
Here are some examples of factorial calculations.

    Public Function fnFact(number) ' a simple cycle example of Factorial function
    Dim tmp As Long ' new temporary variable to keep the "number" variable unchanged
    tmp = number
    fnFact = number
    While tmp > 1
        tmp = tmp - 1
        fnFact = fnFact * tmp
    Wend
End Function

Public Function fnFactR(number) ' a simple example of recursive function for Factorial calculation
    If number > 0 Then
        fnFactR = fnFactR(number - 1) * number ' function calls itself to continue calculations
    Else
        fnFactR = 1 ' function returns {1} when calculations are over
    End If
End Function

Sub Factor_test() 'RUN ME TO TEST ALL THE FACTORIAL FUNCTIONS
    Dim number As Long
    number = 170 ' change me to find Factorial for a different value
    MsgBox "Cycle Factorial:" & vbNewLine & number & "!= " & fnFact(number)
    MsgBox "WorksheetFunction Factorial:" & vbNewLine & number & "!= " & WorksheetFunction.Fact(number)
    MsgBox "Recursive Factorial:" & vbNewLine & number & "!= " & fnFactR(number)
End Sub

All those functions are available to calculate Factorial only for numbers before 170 inclusively, because of large result value.
So for my PC the limitation for WorksheetFunction.Fact() function is also 170.
Let me know, if your PC has different limitation for this function, — it’s quite interesting thing.

UPDATE2
It is recomended to use Long data type instead of Integer each type when integer (or whole number) variable is needed. Long is slightly faster, it has much wider limitations and costs no additional memory. Here are proof links:
1. MSDN:The Integer, Long, and Byte Data Types
2. ozgrid.com:Long Vs Integer
3. pcreview.co.uk:VBA code optimization — why using long instead of integer?
Thanks for @Ioannis and @chris neilsen for the information about Long data type and proof links!

Good luck in your further VBA actions!

The error is here:

If wksht = wkbk.Sheets(2) Then

The Worksheet class doesn’t have a default property, so you can’t let-coerce that object; VBA is trying to invoke the default member of Worksheet to satisfy the comparison operator, but there’s no such member, so error 438 is raised.

I think what you’re trying to do is to validate whether wksht is wkbk.Sheets(2). You need the Is operator for reference equality:

If wksht Is wkbk.Sheets(2) Then

Note that this is different from comparing the .Name of the sheets: here we’re comparing object references.

That said, I’d buy a vowel or two here, it’s too easy to make a typo typing those… Make sure Option Explicit is specified!

Addendum

wkbk.Sheets.Add Before:=wkbk.Sheets(1)
Set DataSheet = ActiveSheet

Worksheets.Add returns the added worksheet object, so you can do this instead, and not rely on the side-effect of the added sheet now being the ActiveSheet:

Set DataSheet = wkbk.Sheets.Add(Before:=wkbk.Sheets(1))