Another >> Unable to set the FormulaArray property of the range class - vba

Question

I've researched as many of the "Unable to set the FormulaArray property of the range class" issues here and in other sites to troubleshoot a FormulaArray operation I'm trying to carry out in a table of data.
I've respected the 255 character limit in my formula taking into account R1C1, I've tried to insert the formula as a text string first. I've tried quite a few things.
Now I'm thinking my issue is with the fact that I'm trying to insert my formula only into blank cells because if I simplify my formula to only be =1+1 I still get the error. If I change .FormulaArray to .Formula to simply enter a standard formula I also get the same error. Is this operation not possible with blank cells?
The error occurs at the .FormulaArray = myFormula1 step.
To confirm, the formula by itself works (pasted further below) when entered into cells manually.
Sub ArrayMacro()
Dim myFormula1 As String
Dim myFormula2 As String
Dim myFormula3 As String
Dim myFormula4 As String
myFormula1 = "=IF(COUNTIFS(MLBtransactions!$D:$D,A$1,MLBtransactions!$A:$A,""<=""&$C2)>0,IF(INDEX(MLB," & "X_X_X)"
myFormula2 = "transactions!$A:$E,MATCH(1,(MLBtransactions!$D:$D=A$1)*(MLBtransactions!$A:$A<=$C2),0),," & "Y_Y_Y)"
myFormula3 = "5)=$A2,""DNP/SUS/MIN"",""with "" & INDEX(MLBtransactions!$A:$E,MATCH(A$1,MLBtransactions," & "Z_Z_Z)"
myFormula4 = "!$D:$D,0),5)),IF(COUNTIFS(MLBstats!$B:$B,A$1,MLBstats!$A:$A,$A2)=1,""DNP/SUS/MIN"",""LEAVE BLANK""))"
Sheets("Sheet1").Select
With Range("E2:AZ140").SpecialCells(4)
.FormulaArray = myFormula1
.Replace ",X_X_X)", myFormula2
.Replace ",Y_Y_Y)", myFormula3
.Replace ",Z_Z_Z)", myFormula4
End With
End Sub
Question update. Using With Range("E2:AZ140").SpecialCells(xlCellTypeBlanks) instead of With Range("E2:AZ140").SpecialCells(4) also ends with the same error.
Here is the full formula being used
=IF(COUNTIFS(MLBtransactions!$D:$D,A$1,MLBtransactions!$A:$A,"<="&$C2)>0,IF(INDEX(MLBtransactions!$A:$E,MATCH(1,(MLBtransactions!$D:$D=A$1)*(MLBtransactions!$A:$A<=$C2),0),5)=$A2,"DNP/SUS/MIN","with " & INDEX(MLBtransactions!$A:$E,MATCH(A$1,MLBtransactions!$D:$D,0),5)),IF(COUNTIFS(MLBstats!$B:$B,A$1,MLBstats!$A:$A,$A2)=1,"DNP/SUS/MIN","LEAVE BLANK"))
Testing with a simplified code (example below) led to the realization as YowE3K points out that myFormula1 needs to be valid in order for the procedure to work.
Sub ArrayMacro()
Dim myFormula1 As String
Dim myFormula2 As String
myFormula1 = "=1+1" & "+2+2"
myFormula2 = "+1+1"
Sheets("Sheet1").Select
With Range("h14:h16").SpecialCells(xlCellTypeBlanks)
MsgBox .Address
.FormulaArray = myFormula1
.Replace "+2+2)", myFormula2
End With
End Sub

Answer 1

When you set a formula using FormulaArray = ..., it needs to be a valid formula. (I think after each Replace the formula needs to continue being valid too, but I haven't tested that. Edit: No, if the Replace would create an invalid formula, it just doesn't process it - but it doesn't crash.)
Your problems all seem to stem from the use of invalid formulas in your myFormula1 variable.
I suggest you use the following:
Sub ArrayMacro()
Dim myFormula1 As String
Dim myFormula2 As String
Dim myFormula3 As String
Dim myFormula4 As String
myFormula1 = "=IF(COUNTIFS(MLBtransactions!$D:$D,A$1,MLBtransactions!$A:$A,""<=""&$C2)>0,IF(1232=$A2,""DNP/SUS/MIN"",""with ""&1233),1234)"
myFormula2 = "INDEX(MLBtransactions!$A:$E,MATCH(1,(MLBtransactions!$D:$D=A$1)*(MLBtransactions!$A:$A<=$C2),0),5)"
myFormula3 = "INDEX(MLBtransactions!$A:$E,MATCH(A$1,MLBtransactions!$D:$D,0),5)"
myFormula4 = "IF(COUNTIFS(MLBstats!$B:$B,A$1,MLBstats!$A:$A,$A2)=1,""DNP/SUS/MIN"",""LEAVE BLANK"")"
'Insert the formula
With Sheets("Sheet1").Range("E2:AZ140").SpecialCells(xlCellTypeBlanks)
.FormulaArray = myFormula1
.Replace "1232", myFormula2
.Replace "1233", myFormula3
.Replace "1234", myFormula4
End With
End Sub

Answer 2

Setting .FormulaArray to a range of cells in VBA is the same as pressing CTRL+SHIFT+ENTER while the entire range is selected. This is used when a single formula is returning an array of results and you want to display that array in the range of selected cells. This requires a contiguous range of cells to display the array. Trying to set .FormulaArray on a non-contiguous range of cells will fail, regardless of whether the formula is good or not.
I think you're trying to create an Array Formula that returns a single result after performing analysis on arrays; and you want this formula used in all blank cells.
In Excel, you would need to array-enter the formula into a single cell and then copy the formula into other cells.
Similarly, you need to do it in 2 steps in VBA. You need to first set the .FormulaArray for just one cell. This will also confirm that the formula is being constructed correctly in VBA. You can then copy that cell to all blank cells, using PasteSpecial if you only want to copy the formula.
Alternatively, you could loop through all blank cells setting the .FormulaArray individually, e.g.:
Dim raCell As Range
For Each raCell In Range("E2:AZ140")
If IsEmpty(raCell) Then raCell.FormulaArray = ...
Next
However, as you have dynamic references, you would need to construct the FormulaArray carefully to correctly determine the formula required based on the .Row and .Column of the current raCell.
Copy and paste would be safer if you have an "achor" point. Somewhere you know you can always enter the exact same formula and get the correct result when copied and pasted to all other cells.