Question

How to get isnumeric function to display an "error" message when necessary.

Sub DetermineinterestSelectCase()
Dim loan As Variant
Dim interest As Double
loan = InputBox("Enter your desired loan amount, from 1 dollar to 5 million dollars.")
Select Case loan
Case IsNumeric(loan) = False: MsgBox ("Error")
Case Is < 1000000: interest = ".08"
Case 1000000 To 4000000: interest = ".10"
Case Is > 4000000: interest = ".11"
End Select
MsgBox ("The loan amount was " & loan & " with an interest rate of " & interest & "")
End Sub

Sub DetermineinterestIf()
Dim loan As Variant
Dim interest As Double
loan = InputBox("Enter your desired loan amount, from 1 dollar to 5 million dollars.")
IsNumeric (loan)
If loan < 1000000 Then interest = ".08"
If loan > 4000000 Then interest = ".11"
If 1000000 < loan < 4000000 Then interest = ".10"
MsgBox ("The loan amount was " & loan & " with an interest rate of " & interest & ".")
End Sub

Answer 1

IsNumeric(s) returns true or false.
It's up to you to handle each case.

If IsNumeric(s) then
MsgBox(msg)
else
do other stuff
end if

Answer 2

Oops - make that

If Not IsNumeric(s) Then
...

to handle the error case.