最好的做法是

Option Explicit

在每个模块的开头。这将确保您声明所有变量并防止大部分拼写错误。
然后，更正这一行：

MsgBox asnwer

你打错了。应该是

MsgBox answer

另外，你不需要将这些变量声明为参数。

Sub area(p As Single, radius As Integer)

将该行替换为：

Sub area()

我还做了一些其他的调整。
最终代码：

Option Explicit
Sub Area()

    Dim answer As Double
    Dim radius As Double

    ' area of circle is 3.14 x r(sq)

    ' Define PI constant
    Dim p As Double

    p = WorksheetFunction.Pi()

    ' Ask for the radius
    radius = InputBox("Enter the radius")

    ' Calculate area
    answer = p * radius ^ 2

    ' Show the user the answer
    MsgBox answer

End Sub

Sub test3()
Dim radius as double
Dim area as double
Radius = 4
Area = 3.14*radius^2
 
'Here the ^symbol is used to raise radius to the power 2

Debug.print " value of radius is: "& radius
Debug.print " value of radius is: "& area
End sub