Unable to get gamma_inv property of the worksheetfunction class error

Iam totally new to the VBA, so I would appreciate your help...

My code so far, first part, where I have made random numbers to the specific range:

Dim i As Long
Randomize
     For i = 1 To 20000
         Range("A" & i) = Rnd()
     Next i
      Range("A1") = ("Rand")
      ActiveCell.Range("A1:A20000").Select
    Sheets("Sheet1").Columns("A").Copy
    Sheets("Sheet1").Columns("B").PasteSpecial xlPasteValues
     Sheets("Sheet1").Columns("A").Delete
    Range("A1") = ("Rand")
      MsgBox ("Nagenerovaných 20 000 hodnôt")

Dim alfa As Integer
  Dim beta As Integer
  Dim a As Long
  Range("I2").Value = InputBox("zadaj parameter alfa")
  Range("J2").Value = InputBox("zadaj parameter beta")
  Range("B2").Select

 Range("I2").Value = alfa
 Range("J2").Value = beta
 For a = 1 To 20000
 Range("B" & a) = WorksheetFunction.Gamma_Inv(Rnd(), alfa, beta)
 Next a

The first part of the code works fine, but it takes a while to make these random numbers(something about 8 secs. on my laptop), but its annoying. Is there another way how to do it?

The second part does not work. What Iam trying to do is that, instead of Rnd() in gamma function, i want to use the random number I have already generated in previous step, but I dont know how to do that. I Have tried the second part of the code with the rand(), because I wanted to know, if its gonna work, like the code itself, so I could modify it.

Iam stacked here for 2 days, glad for any help.

PS: its a school project, the each part of the code is started by pressing a click button, if you are wondering why I have separated it to the 2 parts.

Thanks