Getting 'Runtime error '1004': application-defined or object-defined error' while trying to assign formula to a range

I am getting the "Runtime error '1004': application-defined or object-defined error" when I try to execute the following code, and I have no idea why this is happening.

Dim rangeOfReg As Range
Dim rowOfRegStringfied As String
rowOfRegStringfied = LTrim(Str(rowOfReg))
Set rangeOfReg = Worksheets("Sheet").Range(colOfReg + rowOfRegStringfied)
rangeOfReg.Formula = "=AVERAGEIF(H2:J" + rowOfRegStringfied + ";'Seg a quarta';J2:J" + rowOfRegStringfied + ")"

The value of colOfReg, when it is read is "H", the value of rowOfReg is 20. The error is thrown when this last line is executed.

I have tried not assigning that Range to that variable of type range, but instead calling "Worksheets("Sheet").Range(colOfReg + rowOfRegStringfied)" in the next line but didn't work.
I have also tried replacing commas in the formula being assigned, but the error is still there.

After making the corrections suggested by @chrisneilsen and @freeflow (all are needed), your issue comes down to this piece of the string:

",'Seg a quarta',J2:J"

As you noted, that requires double quotes for

"Seg a quarta"

Inserting a double quote inside a string requires "escaping" it so Excel doesn't interpret it as the end of the string. In VBA, use two double quotes together to escape the double quote:

",""Seg a quarta"",J2:J"

So the full statement will be:

rangeOfReg.Formula = "=AVERAGEIF(H2:J" & rowOfRegStringfied & ",""Seg a quarta"",J2:J" & rowOfRegStringfied & ")"

Excel accepts that without complaining.