What makes you say it's an error output?

Option Explicit

Sub Test()
Dim CatSz As Long, Size As Long, Prop As Double, StdErr As Double
Size = [F23]
CatSz = [F24]
Prop = CatSz / Size
StdErr = Sqr((Prop * (1 - Prop) / Size))

Debug.Print "1", Size, CatSz, Prop, StdErr

StdErr = Round(StdErr, 10)

Debug.Print "2", Size, CatSz, Prop, StdErr

End Sub
1              60            39            0.65          6.15765106730372E-02 
2              60            39            0.65          0.0615765107
That's not an error, just a small number.